Even if i delete the apps and wait weeks/months and then reinstall, i'm logged in again when i first open them.
I have enabled "Settings > Privacy > Advertising > Limit Ad Tracking" and also cleared all Safari history.
How is this possible?
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".
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".
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.)
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).