How do you unscrew a rusted bolt?

There are several techniques.

1. If very rusty clean what you can and then take a torch and heat until you you see it's getting slightly red. Then try to unscrew it right away.

2. If not very rusty use ATF or WD40 or similar sprays on it leave to penetrate for 10-20 minutes then try to unscrew.

And many other techniques but it depend on materials around bolts and also from a quality and material of bolts too.