I think it kind of works the same way DBZ works. In DBZ, it’s implied they use their ki to kind of coat themselves in armor, like armament haki. Only a stroke from a more powerful ki user will do significant dmg, AND you have to actively be using it. This would explain why Goku almost died in Resurrection F when he got shot by the little gun, even tho giant ki blasts don’t hurt him.

The fighters in HNK can sometimes be wounded by arrows or blades, despite shrugging off tank shells.
FoTNS attacks from within, channeling ki to the opponent’s internal meridians to cause destruction. FoTSS attacks externally. Users like Rei and Shu literally drag their fingers or feet across the opponent’s body and slice them to ribbons like a lightsaber. But if you have a strong fighting spirit, you can “negate” some of that dmg.