iPhone 12 completely dead, won't turn on even on the charger

If Finder or iTunes does see it but the screen stays black, try calling the phone or asking Siri something. If it rings or Siri answers, the phone is alive and it's the screen that died. Much cheaper repair than a dead board, and your data is fine.

Try a force restart before anything else, a phone that looks dead is often just frozen with the screen off. Press and let go of volume up, press and let go of volume down, then hold the side button and keep holding until the Apple logo shows, can take 15-20 seconds. If that does nothing, leave it on a wall charger for a good half hour with a cable you know works. A fully drained battery sometimes won't show anything for a while. Also look in the charging port with a torch, pocket lint packed in there stops the cable seating properly. Still nothing after that? Plug it into a computer and see if it shows up in Finder or iTunes. If it does you can try a recovery, if it doesn't it's likely a repair job.