I installed GrapheneOS on a spare Pixel 4a recently...through a browser window. I thought at first that I had to download a firmware flasher or something, but no, it updated the device from that webpage! That was impressive.
The other thing I would like to mention: Chromium is installed, so PWAs can be installed instead of connecting to a sandboxed google play or something. The PWAs I tried looked just like on an iPhone or desktop. Yes, we are far, far away from PWAs being a complete replacement. But it is in theory possible:
An independent mobile OS with platform-independent apps. No Apple/Google ID needed, no appstores. That was the point of this test install, and it worked.