The biggest thing that takes up storage is photos. If you're running iOS 9 or later, then go to Settings -> iCloud -> Photos and enable iCloud Photo Library. Then, make sure optimize iPhone storage is checked. Also, delete any apps that you may not use. Pay close attention to games. They eat up the most space. Switching to a streaming music service like Music or Spotify will also let you free up space by deleting music on your device.
Answer from CSstudent on Stack ExchangeVideos
How do I reset my iPhone to clear system storage?
Do deleted photos take up storage?
Why is my photo storage so high when I have no photos?
So I own a 128Gb iPhone 12 mini. Have experienced the same issue with my prev iPhone as well. My storage got full almost three days back. Now I deleted an app taking around 1.5 gb of space and then it got emptied as expected. But after sometime what do I see, the storage is full again with me not doing anything. Next I uninstall another app, storage empties and then it is full again… What the….
Now I delete some pictures and clear my bin too almost 2 gb.. After some time the storage is full again.. Don’t know why and what is happening, does this happen with you guys too ?
And also , how do you deal with the issue once your storage is full…
The biggest thing that takes up storage is photos. If you're running iOS 9 or later, then go to Settings -> iCloud -> Photos and enable iCloud Photo Library. Then, make sure optimize iPhone storage is checked. Also, delete any apps that you may not use. Pay close attention to games. They eat up the most space. Switching to a streaming music service like Music or Spotify will also let you free up space by deleting music on your device.
You mean your iPhone storage is full and you you want to back up some data on iCloud so that you can have more storage on your iPhone, am I correct?
Now, I'd like to share with you how to turn on and use iCloud.
Go to Settings → iCloud to see all the settings and options on your iPhone or iPad. If you have not turned iCloud on already this is where you can log in to iCloud with your Apple ID and password. After you log in you will see options to turn on iCloud options which control what iCloud can do. Most users will want to leave these iCloud options turned on. When turned on, the options sync to iCloud so that they are accessible on iCloud.com and on your other Apple devices.
You can toggle any options you don’t want to the off position to stop syncing that data with your devices and iCloud. Any of the settings with an On > option offers more information and settings that you can control. These include iCloud Drive, Photos, Backup, Keychain and Find My iPhone.
If you want to know how to make more space, you can try other ways, such as clean up caches and temporary files, maybe iMyfone Umate is a good choice for you.
Have you installed the app "WhatsApp"? Seems it was the app "WhatsApp", creating lots of cache files and stealing storage from the phone. You should try to install the update and see whether the storage came back to normal.