There is F-Droid, which is a terrible client app but provides (non-Amazon, non-pirate) open source software such as NewPipe, Simple Notes or DavX.

Foxydroid is a really nice and imo much faster F-Droid frontend https://f-droid.org/en/packages/nya.kitsunyan.foxydroid/

(not affiliated just a big fan)

There's also Aurora Droid, an F-Droid client developed by the creator of Aurora Store.

https://f-droid.org/en/packages/com.aurora.adroid/

It's currently being rewritten to match Aurora Store v4's interface.

I think that these demands should require the built-in app store to support an interoperable feed to other sources. I think that they do but not clearly enough.

I doubt Google would interpret it that way, but that would be the fair way to do it. Make it so that the information is interoperable rather than just not actively disallowing it. Then it can stop being a monopoly.

Fdroid is ok. Not the best, but it does function.

WeChat is pretty much an app store (among many other things).

Apkpure and fdroid.