Stark#GooglePlay services are separate from the #Ansroid install. Android is an #opensource operating system that you can actually download and extend. This base version doesn't include GooglePlay services.
Imagine it like clean #Linux and then versions of Linux made for a specific service like a #Mastodon server. The base layer is the same. The Mastodon image just has some stuff pre-installed.
So, the Linux and Mastodon code will be updated separately.
As for why you need to manually check for updates. Well, that's easy. That's just bad design and implementation by #Google, as we've come to expect from them by now.