How the heck am I supposed to scan a QR code on my phone screen?
Scan Barcode and QR code from the mobile screen itself - Android Enthusiasts Stack Exchange
How Do I QR Scan from my Android Phone (Your Phone App)?
How the hell are you supposed to scan a QR code on an already taken picture?
Videos
be me.
i go to a website on my phone and there is a QR code
i screenshot it and mail it to my computer
then I turn on my camera on my phone and scan the monitor of my computer
there must be a better way
As already described in the above comments, this would require taking a screenshot and saving it as an image file. Having done that, there are several barcode scanners available in the Play Store supporting "loading" of barcodes from image files, as e.g. QR BARCODE SCANNER. QR Droid also seems to be capable of what you're looking for:
Scan QR codes from your camera, browser, SD card, or saved image
An open-source, ad-free alternative QR & barcode scanner and generator is SecScanQR. It's free, high-quality, and user-friendly.
Though there is no button to load images within the app itself, sharing an image to it from any other app, e.g. your photo gallery, will make it scan the image for QR codes. If the QR is on a website or other app, taking a screenshot an immediately sharing that to SecScanQR works perfectly. The result is the same as if you had just scanned it in the app.