film techniques - How do they shoot fight scenes without causing harm to performers?

For the old John Wayne fights, they coordinated a swung punch just missing with the recipient moving their head quickly as if they were hit. Sound effects fill the credibility gap.

Sometimes in comedy shows, the person being hit will move their head the wrong way.

In modern digital action scenes, there can be wires, harness and such which are removed from the image later.

Also an actors face can be digitally added to a stunt performer. In Titanic, in the scene where our heroes run away from the gushing water, there was digital face replacement.

Whenever characters run from an explosion or fireball, it is always chromakey. It's actually easier to do it that way as well as safer.

Don't forget, there is such a complex set of insurance, Producers can't put a star at risk without the insurance company approving, otherwise a dead or injured star costing a hundred million or more might come out of the Producers own pocket. Double ouch!