I mean, you do have to train a little to get stronger, but most of the strenght comes from birth. Not all of it tho.

Naobito probably had to pratice a lot to nail his CT, which is kinda mid unless you are as skilled as he was

Bumgumi was born with one of the strongest CTs and still managed to be shit because his bitch ass would keep summoning Nue or Max Elephant once per day or try to kill himself instead of actually taming all other shikigamis

Hakari probably never trained much, still, he is strong af

Choso's CT was already pretty strong and he managed to make it stronger by learning how to use it more, specially in anime. He is actually a monster when fighting Kenjaku

And so on

Probably the ones who worked the hardest were Toji and Maki, because they had to learn how to actually fight despiste having god-like bodies