How will Google enforce the new requirements for apps outside Play Store to register? will they implement it in Google Play Services or in Android itself?

I think it’ll be a Google Play Services thing, though I’ve also heard it’d be into other app stores that Google allows in AOSP.

I would expect through changes to the Android operating system’s core functionalities and APIs. It’ll be a lot easier to enforce all this when handled at the OS level. I could be wrong though.