For many many years now Google is adding all non-os functionality into "support" libraries, "Google Services" - the name changes periodically - but the essence is the same: Making sure that any Android that is not "Google certified" won't be able to run Android apps - which all use these libraries - and won't have apps such as YouTube, Gmail, Photos come pre-installed..
I believe many of the microg services are mocked out in a way that makes apps 'work', but complex things like step counting in Google fit or face recognition will always return some bogus results.
Technically Android is OpenSource.
De-facto - its not.
For many many years now Google is adding all non-os functionality into "support" libraries, "Google Services" - the name changes periodically - but the essence is the same: Making sure that any Android that is not "Google certified" won't be able to run Android apps - which all use these libraries - and won't have apps such as YouTube, Gmail, Photos come pre-installed..