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Google originally did not want to make any money from Android's app store
(www.androidauthority.com)
This is a most excellent place for technology news and articles.
Real talk: sideloading is allowed on android in the most maliciously compliant way possible.
Google restricts what other app stores can be included with devices that ship with play services
User-sideloaded app stores can't auto-update apps
Play protect will flag any app that the play store has hashes of, but was installed by another app store. (Developers cannot, for example, upload a list of valid hashes for their apps to Google to prevent false positives here, effectively making other install routes appear as malware if they're different.)
I think they've changed this in recent versions. While I disable auto-update everywhere, Neo Store (my f-droid app) has a setting for auto-update.
My phone didn't come with Google play services, I installed them myself