Grandpa taught Goku Martial Arts to the point where he was able to surpass a Shaolin Monk who dedicated his life to the arts.
Everyone else taught him how to get the most out of the skills his Grandpa taught him.
And pretty much every DB fight is raw skill. It's not like a Superman fight where the jokers lead off with their chins and take blows to the head, they actually parry and counterstrike every single blow accordingly with lightning quick reaction.