[inapis]

I believe they store some data in your iCloud account. Uber used to do this too. The only way to get rid of it is to sign out and then again explicitly ask to delete account data on this device, then uninstall and reinstall to use the service without signing in.

[v3v3]

What Google does is store account data that is shared with all Google apps under "Google LLC".

Go to "Settings > General > iPhone Storage" and you will find "Google LLC" where it explains the data stored is shared with all Google apps.

Microsoft does the same thing to enable you to quickly sign-in to their different apps under the name "Microsoft Corporation".

[DavideNL]

Hmmm, there is no "Youtube" or "Google Home" item in "Settings > AppleID > iCloud" (where you can enable/disable iCloud access for specific apps.)