This answer is summarized below.

• MBR partition a 16 GB or larger flash drive to contain a FAT32 volume and an ExFAT volume.
• Copy all the files, except sources/install.wim, from the mounted Windows ISO file to the FAT32 volume on the flash drive.
• Copy sources/install.wim from the mounted Windows ISO file to the ExFAT volume on the flash drive.
• Boot from the flash drive and enter a command to split the install.wim on the ExFAT volume to two files placed on the FAT32 volume.
• The result is an USB flash drive which can be used to install Windows 11.

The steps are given next.

1. Goto Microsoft's Download Windows 11 website and download the Windows 11 disk image (ISO) for x64 devices. In my case, the Win11_24H2_English_x64.iso file was downloaded to my Downloads folder. If your file name is different, then make the appropriate substitutions when entering the commands below.

   Insert a 16 GB or larger flash drive in an USB port on or connected to the Mac.

2. Open a Terminal application window.

   Use the command below to determine the identifier for the USB flash drive. I my case, this was disk2. If your identifier is different, the make the appropriate substitutions in the following commands.

   diskutil list external
   

   Enter the command below to format the flash drive.

   diskutil partitiondisk disk2 mbr fat32 MYFAT32 8G exfat MyExFAT R
   

   Enter the command below to remove all extended attributes from the Windows ISO file. This will remove the quarantine placed on the file when it was downloaded.

   xattr -c ~/Downloads/Win11_24H2_English_x64.iso
   

   Enter the command below to mount the Windows 11 ISO file. In my case, the output from the command shows the file was mounted on /Volumes/CCCOMA_X64FRE_EN-US_DV9. If your folder name is different, then make the appropriate substitutions when entering the commands below.

   hdiutil attach ~/Downloads/Win11_24H2_English_x64.iso
   

   Enter the command below to change the current directory to /Volumes/CCCOMA_X64FRE_EN-US_DV9.

   cd /Volumes/CCCOMA_X64FRE_EN-US_DV9
   

   Enter the commands below to copy the files from the mounted ISO to the flash drive. Note: Since the install.wim file is very large, the cp command can take a while to complete.

   rsync -r -t --exclude=sources/install.wim . /Volumes/MYFAT32
   cp sources/install.wim /Volumes/MyExFAT
   

   Enter the commands below to unmount the Windows 11 ISO file.

   cd ~
   hdiutil detach /Volumes/CCCOMA_X64FRE_EN-US_DV9 
   

   Enter the command below to eject the flash drive. Afterwards, you can remove the flash drive.

   diskutil eject disk2
   

   You can quit the Terminal application.

3. Using the computer where Windows is to be installed, UEFI boot from the flash drive. You should get a window similar to the one shown below. You can ignore the contents of this window.

   

4. Press the shift+F10 key combination to open Command Prompt window.

   Enter the command below. From the output, determine the drive letters for the MYFAT32 and MyExFAT volumes. In my case, the was C and D respectively. If your letters are different, the make the appropriate substitutions for c:\ and d:\ when entering the dism command given below.

   echo list volume | diskpart
   

   Enter the command below to create the install.swm and install2.swm files in the c:\sources folder. Note: This command will take a while to complete.

   dism /split-image /imagefile:d:\install.wim /swmfile:c:\sources\install.swm /filesize:4000
   

   Enter the command below to close the Command Prompt window.

   exit
   

5. Close the window. When the popup shown below appears, select "Yes". The computer will reboot.

   

   The next time you UEFI boot from this USB flash drive, you should encounter the following window. You then can proceed and install Windows 11.

   

Example of Using the Terminal Window

Last login: Sun Mar  9 09:16:57 on console
davidanderson@Davids-Mac ~ % diskutil list external
/dev/disk2 (external, physical):
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:     FDisk_partition_scheme                        *15.9 GB    disk2
   1:                 DOS_FAT_32 ⁨MYFLASHDRV⁩              15.9 GB    disk2s1

davidanderson@Davids-Mac ~ % diskutil partitiondisk disk2 mbr fat32 MYFAT32 8G exfat MyExFAT R
Started partitioning on disk2
Unmounting disk
Creating the partition map
Waiting for partitions to activate
Formatting disk2s1 as MS-DOS (FAT32) with name MYFAT32
512 bytes per physical sector
/dev/rdisk2s1: 15594504 sectors in 1949313 FAT32 clusters (4096 bytes/cluster)
bps=512 spc=8 res=32 nft=2 mid=0xf8 spt=32 hds=255 hid=2048 drv=0x80 bsec=15625000 bspf=15230 rdcl=2 infs=1 bkbs=6
Mounting disk
Formatting disk2s2 as ExFAT with name MyExFAT
Volume name      : MyExFAT
Partition offset : 15628288 sectors (8001683456 bytes)
Volume size      : 15370240 sectors (7869562880 bytes)
Bytes per sector : 512
Bytes per cluster: 32768
FAT offset       : 2048 sectors (1048576 bytes)
# FAT sectors    : 2048
Number of FATs   : 1
Cluster offset   : 4096 sectors (2097152 bytes)
# Clusters       : 240096
Volume Serial #  : 67cdc143
Bitmap start     : 2
Bitmap file size : 30012
Upcase start     : 3
Upcase file size : 5836
Root start       : 4
Mounting disk
Finished partitioning on disk2
davidanderson@Davids-Mac ~ % xattr -c ~/Downloads/Win11_24H2_English_x64.iso
davidanderson@Davids-Mac ~ % hdiutil attach ~/Downloads/Win11_24H2_English_x64.iso
/dev/disk3                                              /Volumes/CCCOMA_X64FRE_EN-US_DV9
davidanderson@Davids-Mac ~ % cd /Volumes/CCCOMA_X64FRE_EN-US_DV9 
davidanderson@Davids-Mac CCCOMA_X64FRE_EN-US_DV9 % rsync -r -t --exclude=sources/install.wim . /Volumes/MYFAT32
davidanderson@Davids-Mac CCCOMA_X64FRE_EN-US_DV9 % cp sources/install.wim /Volumes/MyExFAT
davidanderson@Davids-Mac CCCOMA_X64FRE_EN-US_DV9 % cd ~
davidanderson@Davids-Mac ~ % hdiutil detach /Volumes/CCCOMA_X64FRE_EN-US_DV9
"disk3" ejected.
davidanderson@Davids-Mac ~ % diskutil eject disk2
Disk disk2 ejected
davidanderson@Davids-Mac ~ % 

Example of Using the Command Prompt Window

Microsoft Windows [Version 10.0.26100.1742]
(c) Microsoft Corporation. All rights reserved.

X:\sources>echo list volume | diskpart

Microsoft DiskPart version 10.0.26100.1150

Copyright (C) Microsoft Corporation.
On computer: MININT-NBNQP5I

DISKPART>
  Volume ###  Ltr  Label        Fs     Type        Size     Status     Info
  ----------  ---  -----------  -----  ----------  -------  ---------  --------
  Volume 0     E                       DVD-ROM         0 B  No Media
  Volume 1     C   MYFAT32      FAT32  Removable   7629 MB  Healthy
  Volume 2     D   MyExFAT      exFAT  Removable   7505 MB  Healthy

DISKPART>
X:\sources>dism /split-image /imagefile:d:\install.wim /swmfile:c:\sources\install.swm /filesize=4000

Deployment Image Servicing and Management tool
Version: 10.0.26100.1150

The operation completed successfully.

X:\sources>exit

OK. Here are some instructions from Gizmodo that will put the ISO onto the flash drive with a Mac... they won't make it bootable though... read on...

1. Open a Terminal (under Utilities)

2. Run diskutil list and determine the device node assigned to your flash media (e.g., /dev/disk2)

3. Run diskutil unmountDisk /dev/diskN (replace N with the disk number from the last command; in the previous example, N would be 2)

4. Execute sudo dd if=/path/to/downloaded.iso of=/dev/diskN bs=1m (replace /path/to/downloaded.iso with the path where the image file is located; for example, ./windows7.iso)

5. Run diskutil eject /dev/diskN, and remove your flash media when the command completes (this can take a few hours on slower drives)

Now... after you read all that, on the Gizmodo page it says that if you want to make it bootable you should use a utility called Live USB helper they link to (which isn't there any more) and use a Mac mounting tool (which isn't there either!) to force the ISO to mount on the Mac so you can copy the files over. So... that article may prove useful, but probably not. I can find other copies of Live USB helper out there, but they are all Windows executables.

One method that will work is to swap the hard drive out of the PC and into the Mac. Put your Windows 7 installation disc into the Mac optical drive, and install Windows 7 on that drive. Once it is up and running (don't worry about drivers yet), follow these steps.

1. Run Command Prompt as administrator
2. Type to Command Prompt: %windir%\System32\Sysprep\Sysprep.exe, and hit Enter
3. In sysprep dialog that opens, choose “System Cleanup Action” as “Enter System Out-of-Box-Experience (OOBE)”, select “Generalize”, and select “Shutdown Options” as “Shutdown”. Click “OK”
4. Sysprep generalizes now your Windows 7 setup and shuts down your computer. Do not run any other programs during this phase!
5. Remove the drive from your Mac. Put it back into the PC.
6. Boot the PC from sysprep generalized hard disk. You will notice Windows booting as if it was the first boot after installation, installing default and updating registry. One or two reboots are needed, depending on your system specifications
7. When Windows finally boots up, you will need to enter all information as if this really was a new, fresh installation

All that is from here, but it applies in this situation a well, since you would be moving a Windows 7 installation from one computer to another. Considering that trying to make a Windows bootable USB stick with a Mac requires tools that either don't exist any more or are not being developed any more... this second method will most likely prove more dependable.