Launcher, Phone app, Messages, Contacts, Calendar. All proprietary.
Notification API, Location API, SMS APIs and step counter APIs, and even the new app format. All proprietary.
It's a huge undertaking building an app for Android today without using Google's services, and sometimes even impossible.
If you want to verify a user by sending a code through SMS (think Whatsapp sign-up) you either have to use the proprietary API, or you'll be banned from the play store. Using the open source API instead gets you banned because it allows you to read all SMS — and Google never cared to build a new open source API because they already have their proprietary API anyway.
And nowadays you don't have APKs anymore but often app bundles, a special format which only the Play Store can parse.
And if you create a new app today, per default you don't even get the signing key to publish APKs yourself, but only Google gets to keep it.