Also afaik F-Droid insist on building apks directly from source, so non FOSS apps basically can never use it even if they are ok with showing the tracking they do.

They can, but they'd need to run their own repository.

F-Droid makes it easy to add other repos.

https://f-droid.org/tutorials/create-repo/

https://f-droid.org/tutorials/add-repo/

even if this was true, you can make the app fully oss but most of the main logic would be handled by the server

Why are we even discussing F-Droid here? The topic at hand is whether Google has a monopoly on the Android app market, not whether anyone can put their app on F-Droid, and there are plenty of alternative app stores besides F-Droid.

Why is it a monopoly? Can't you install other app store?

Sorry if that was unclear, I meant that they don't. I'm just confused as to why we're talking about F-Droid specifically.

Setting up your own F-Droid repo is quick and easy, I've done it myself. Adding the repo can be done by copying the link or scanning a QR code.

I'm sure companies would prefer a platform where they can pay money to get featured on the front page which comes with their repo preloaded, but if you're a small app business, hosting an F-Droid repo for test releases and perhaps releases for non-Google phones, you can get rid of the auto-updater if you just point people at F-Droid.