Despite that we (I) know next to nothing about it, neither on the user nor the technical side, so a bunch of deep dives would be welcome.
Google has kept an infamously tight leash that OEMs must not stray too far from; the Open Handset Alliance ensures certain Google apps come pre-installed, down to homescreen placement. The OHA mandates OEMs not release "incompatible" versions of Android, or other mobile OS', as well. Should an OEM want to sell Windows Mobile, Amazons FireOS, or Firefox's mobile OS, they will likely lose their license to sell anything with the brand name Android.
Google has also been moving away from the 'O' in AOSP for some time as well. Running many AOSP apps means dealing with what Google treats as abandonware, such as the eMail app, Contacts, and the open launcher (replaced with Google's proprietary launcher).
I'm certain I don't have to tell this crowd about the death of bootloader unlocking and the ROM scene. Telling me this isn't pushed by Google (which I agree with), and following up with "And Google has no way to prevent this" isn't something I see as believable. Google mandates where the YouTube and Chrome apps get placed on the homescreen; you're telling me that, in order to be licensed as Android, Google can't similarly mandate bootloader unlocking?
Nothing changed last week, or even the week before, but the direction isn't terribly difficult to see, IMO.
Some initial versions of HarmonyOS was partially open source, the "NEXT" version isn't.