Supposedly the compatibility is via a Wi-Fi spec that Apple was forced to switch AirDrop to which likely means that support for the spec needs to be implemented on the actual chipset.
This is a bit out of my realm of knowledge, but that would be my assumption. Kind of like BLE when that first became a thing.
Do you mean the call screening feature that Pixels have? When my Pixel does a call screen, it does announce to the other user that the call is being recorded.
I mean technically, yes. However the sheer amount of LoC chromium has and the costs of actually hard forking (and properly maintaining it) makes it quite difficult. That's why right now we only have the choice of Firefox based browsers and Chromium, then hopefully a good third contender being the Ladybird browser in the future.
You could also go build a house (or even a cabin) with your own two hands, but most people typically go and buy one or pay for one to be built for them instead.
Yeah, I think they're thinking of SteamOS which has a
steamos-readonlycommand which can be used to disable immutability (though, changes of course get wiped upon a system update).