And there is an entire vendor partition (On the Pixel 3a, it was >400 MB). However, even if a user built AOSP with those drivers, VoLTE, SMS on LTE, Wi-Fi Calling, eSIM, and a few other things (Sorry I don't remember) do not work.
To get those features, one has to extract them from a stock image:
https://source.android.com/setup/build/downloading#downloadi...
https://developers.google.com/android/drivers
And there is an entire vendor partition (On the Pixel 3a, it was >400 MB). However, even if a user built AOSP with those drivers, VoLTE, SMS on LTE, Wi-Fi Calling, eSIM, and a few other things (Sorry I don't remember) do not work.
To get those features, one has to extract them from a stock image:
https://wiki.lineageos.org/extracting_blobs_from_zips
I can't imagine this is any better for newer Pixel phones, nor for any non-pixel phones. This will be true for compiling AOSP or any Android ROM.