Privacy question: What privacy is exposed when you set Android to a Google Account?

[paul]

Privacy question:
*What privacy is exposed when you set Android to a Google Account?*

As most must know, Android works perfectly fine _without_ ever setting the
phone to any account, let alone a Google Account, which is a privacy boon.

Notice you can still _have_ as many Google Account as you want to have; and
you can still access your email; and your youtube subscriptions; and you can
still download apps from the Google Play repository; and you can still use
the Google search engines; and you can still use Google Maps; and you can
do real-time calendaring; and you can still access your contacts; and you
can still easily backup/restore almost everything on the phone; & you can
can still update your OS, etc., *all without setting Android to an account!*

There's nothing stopping you from using any given app (e.g., a mail user
agent, or a YouTube client, or a Google Play client, etc.) with or without a
Google Account, but the key difference here is whether or not the Android
operating system is set to log into that account (usually upon startup?).

Important privacy questions arise when comparing the privacy of Android
phones set up to log into Google Accounts versus those that aren't.

When exactly does Android log into your Google Account anyway?
And what activities are thereafter exposed to Google by doing so?

*Do you know what is exposed when you set Android to a Google account?*