Does leveling up make my attacks stronger?

When my trainer levels up, does that directly cause my attacks or defenses to become stronger? Or do attacks and defenses only become stronger by powering up Pokémon?

I understand that my trainer leveling up allows me to power up my Pokémon further, but what I’m asking is if my trainer levels up but I don’t power up any Pokémon will my attacks and defenses be stronger than they were before leveling up, or if powering up Pokémon is the only thing that makes attacks and defenses stronger.