On Android phones you can just toggle a setting to allow apps from outside Google Play and then install whatever you want however you want. You can e.g. send your program over FTP or Bluetooth and click the APK file to install it. Or you can use F-Droid with a repository of your choice: https://f-droid.org/

Note that the permissions system, app signing (with any key as long as it's the same for subsequent versions of the app) and other safety features are built into the system so they still work the same - security it not dependent on the Play Store.

You can install other app stores on Android. Amazon for example has one you can install. I believe it requires enabling app side loading, but it is perfectly possible.

Well, whether it is or not, why does that matter?

"Google does it too" isn't a valid defense if it does turn out Apple is found to have done something wrong.

Adding to the existing responses, there's also F-Droid, which specializes in being an "app store" for free-and-open-source Android apps: https://f-droid.org/

just a guess... google does not hold a monopoly over app selling in the android program space. there's the amazon store and i'm pretty sure one of the chinese hardware android companies have one.