Even if you delete and reinstall the app, it remembers your logins and who you are. Even after you wipe the iPhone/iPad and setup with the same Apple ID/iCloud account.

Discord automatically logged me in ON A NEW IPAD, because I signed into the same Apple ID/iCloud account.

and I assume all the other apps from a company can share that info, like Facebook ↔ WhatsApp

Funnily enough, Apple allowed users to delete that data when deleting an app, during an iOS beta, but removed that feature before release, heavily in favor of the spyware and against users.