> The core is an essential part of an operating system, but useless by itself
though I think this is a bit exaggerated, no? AOSP gave me a perfectly fine, modern Android 10 experience
I've tested using microG in order to gain some functionality which is missing on pure AOSP builds (eg push notifications) and even with their FOSS minimal implementation of Google Play Services there's still subpar functionality like being unable to use many apps with integrated maps (Uber, Tinder, Google Maps).
Avoiding Google on your Android device results in an undeniably degraded user experience still. They additionally have abandoned many of the AOSP apps like the Calendar and Keyboard, to instead develop and maintain their own branded versions. F-Droid, LineageOS and microG in particular are picking up the slack, but we're still a good way from having the same level of functionality whilst avoiding Google proprietary software.
What you’re refering to as Android, is in fact, Google Play Services/Android, or as I’ve recently taken to calling it, Google Play Services plus Android. Android is not an operating system unto itself, but rather another component of a fully functioning Google Play Services system.
Many mobile users run a modified version of the Google Play Services system every day, without realizing it. Through a peculiar turn of events, the version of Google Play Services which is widely used today is often called Android, and many of its users are not aware.
There really is a Android, and these people are using it, but it is just a part of the system they use. Google Play Services is the core: the program in the system that is required for full functionality of a mobile device. The core is an essential part of an operating system, but useless by itself; it can only function in the context of a complete operating system. Android is normally used in combination with Google Play Services: the whole system is basically Google Play Services with Android added, or Google Play Services/Android. All the so-called Android ROMs are really distributions of Google Play Services/Android!