Some time ago I installed a certain social network app and logged in using my phone number. Then I uninstalled it.
Today I reinstalled it, and when I first launched it, I noticed that the phone number field is already filled with my number. I tried uninstalling and reinstalling it, and it still remembers my number.
I uninstall the app, it doesn’t say that the app keeps data in iCloud, and I don’t see that app in “Manage my storage” menu.
The app is installed on iPad without cellular module. The phone number is not linked to Apple ID I use on this iPad. The previous install was on the same device I think.
When I install it on iPhone which actually has the SIM card with that phone number, it doesn’t show up in the login menu. I think I didn’t install the app on that phone before.
I tried various internet connections for the iPad when I first launch the app after the reinstall, and the phone number still shows up.
When I disconnect from the internet on the app’s first launch, it still shows the phone number.
So how does the app remember the number? I thought it could access some device id and link it to my number in their servers on first launch, but the launch without internet connection still shows it.
The information is stored in your Keychain. This information persists even when you delete the app and its data files.
You cannot see these entries when you look at Settings > Passwords. There's no Apple provided user interface for looking at or removing these entries from the Keychain.
If you want to get that information specifically removed, you'll need to use functionality from the maker of that social networking app. Apart from that you can of course erase the phone completely (Settings > General > Reset > Erase all content and settings), but that would also delete every other apps and their data. If you use iCloud Keychain syncing, you might need to clear that keychain as well.