I just installed GrapheneOS on a Pixel 6, and am stumped at a step I thought wouldn't give me trouble: I can't figure out how to install an .apk file that I've copied to the phone. If I go to Files --> Downloads, long press the .apk and select "open with," nothing happens.
I've set Settings --> Apps --> Special App Access to "Install unknown apps," but that didn't help.
I had no trouble installing apps from a Pixel 4A with GrapheneOS. FWIW, I think that this 4A handset started out with Android 11, then was upgraded to Android 12 through GrapheneOS updates.