or how to un-stick a stuck screw

If your screwdriver head is old and worn, grind it square so it fits exactly.
A sharp tap with a hammer can sometimes jar the screw free.
Try doing the screw up a half-turn to loosen its grip.
Apply heat with a big soldering iron
Use a ratchet handle or impact screwdriver
If all else fails, drill the head off and work the rest out with a pair of grippers