On general aviation planes there are usually two altimeters that are independent. The instrument a pilot uses to fly with (round dial gauge or digital version in glass cockpit) and one inside the transponder itself. If you lose your altimeter you actually can call up atc if you have a mode-c transponder and have them read it to you. It’s separate.
No idea how it works on a passenger jet, but I would be shocked if it was different.
No idea how it works on a passenger jet, but I would be shocked if it was different.