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