Pixel is running something close... But as for what their adding? All kinds of stuff, from camera features to launchers to subtle tweaks

But the reason no one is running stock android is the reason the Linux phone project is so difficult... The drivers

Your phone is basically running two sets of firmware. One controls the radio, coordinates with cell towers, handles data both as an Internet connection and a set of old cell protocols we still use sometimes (like phone calls and public alerts), plus hardware backdoors.

And then there's Android, which has some degree of control over that mess, but it has to be custom fit to each black box or you get bugs, because they're all different

The only way to run stock android is through an emulator, and even then most developers just use the pixel builds