This may have changed recently, haven't used an Android in a bit, but this wasn't true (and still probably isn't).
Specifically my company wanted to break our app into multiple apps and give a way for people to easily install the companions. If you send the user to the Play Store page, about 75% of users would dropoff, in user studies much of it was confusion about what to do once they saw the play store page.
Google has an api that is a much simpler flow than the full store experience which has much lower user dropoff. If you're curious where it's used, one example is the Google Drive app to install the Docs, Sheets, etc.
The API validates that the calling app is an app signed by the Google dev cert.
This isn't a security issue, you could just ship one app with all the functionality. It's clear they just wanted to reduce user drop off in a flow, but now provide that option to others.
I understand the potential for abuse even though the api still checked with the user. It wouldn't bother me if no one had access to the API, we'd just have to deal with it. But it felt anti-competitive that Google had it and we did not.
IIRC there are a few other similar non-security private APIs and they also felt anti-competitive to us. That said, it was no where near on the order of Apple, where it's commonplace on iOS.
I believe that API is in the play store itself. If your user used a third party store, you could ask them to give you an API to install other apps with no user interaction too.
As far as the platform is concerned, any app with the right permissions can install any other app. It just so happens that only the play store and other stores have that permission.
That's true; you're right that there is a open platform between the AOSP layer and the APK apps that run on top of it.
As you point out there is a fly in the ointment (which is enhanced by Google making it very difficult to ship with anything but the Play Store).
As an app developer like Spotify, the platform is the base of devices out there in the wild and how I build on top of them which, for me, is inclusive of the play services and play store.
Android definitely is a lot more open than iOS. But when the Play Store enforces a mechanism that favors Google, it's not of a practical difference to my livelihood than when the App Store has policies that favor Apple. If you have a really loyal following, like Epic/Fortnite, it's better than nothing. But even then the barrier to your users is huge and even the most establish companies with technical user bases will have a hard time crossing it. Nevermind the fledgling startups or developers with non-technical user bases.
Specifically my company wanted to break our app into multiple apps and give a way for people to easily install the companions. If you send the user to the Play Store page, about 75% of users would dropoff, in user studies much of it was confusion about what to do once they saw the play store page.
Google has an api that is a much simpler flow than the full store experience which has much lower user dropoff. If you're curious where it's used, one example is the Google Drive app to install the Docs, Sheets, etc.
The API validates that the calling app is an app signed by the Google dev cert.
This isn't a security issue, you could just ship one app with all the functionality. It's clear they just wanted to reduce user drop off in a flow, but now provide that option to others.
I understand the potential for abuse even though the api still checked with the user. It wouldn't bother me if no one had access to the API, we'd just have to deal with it. But it felt anti-competitive that Google had it and we did not.
IIRC there are a few other similar non-security private APIs and they also felt anti-competitive to us. That said, it was no where near on the order of Apple, where it's commonplace on iOS.