Android is opensource. No Google needed there.
It's a tool, not a cage.
GrapheneOS for example...
Discussion
Why does it need to go on the Google App Store then to download? Genuine question, I have never had a android and I'm not very technical ?
It does not need to go there. On Android you can install app by just downloading APK file and install it.
Of course there is a risk that you download harmfully app, but that's a up to you to decide (by default in settings the installation from unknown sources is disabled). Also there is a problem with updates, some apps have bedded update mechanism some don't. So if you install APK you have ti update manually.
Also there is F-Droid. It's an alternative store for apps. There are more stores like it (I think Amazon has its own), but F-Droid is most known on the Open-Source scene.
So back to putting Amethyst on Google Play Store. It is just a convinience for ordinary people who don't know/want F-Droid or download APK.
And again, it a choice. It's not necessarily bad to download app from Google Play Store (it's possible to do it even on ungoogled Android via proxy-app called Aurora). But you have that choice.
In Apples ecosystem, you don't have that choice. It's a golden cage.