The use IDFV(id for vendor) for this. It's an identifier that is shared between all apps of the same vendor.

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.

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".

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

Go to "Settings > General > iPhone Storage".

On the list of apps you will see "Google LLC". This is where Google stores account data and this is shared with all Google apps.

Under Google LLC it says "Data shared by X apps".

Nope, unfortunately there's not a single item in there containing/related to Google :'(

You need to have at least 2 apps installed from Google for it to show up.

I installed the Youtube and Google Home apps.

However, still i see no "Google LLC" in "Settings > General > iPhone Storage".

(i do see Youtube and Google Home. But after deleting them and reinstalling Youtube i am automatically logged in again.)

If you go to Settings > Passwords and Accounts, are you signed into a Google account there?

No... only iCloud and my Mail account (strato.de).

I just noticed something else though: when i drag down from the home screen and enter "google" in the search bar, the result list displays a "Settings" item named "Google Home" with the gray ios-settings icon next to it. If i click it to open it, the settings app opens but it doesn't show me anything specific, just the main setting screen.

Note that Google Home is not installed on any of my devices (i removed them a long time ago after ditching the Chromecast).

Do you have any other google apps installed in your phone? Apps from the same developer can share information.

No... I don't have any Google app installed.