whenever i see a fully modified 250r,they always have a metal box with like stickers on it on each side of the front fender, what is that

I think you mean the radiator scoops. They are mounted on both sides of the radiator to get more air flow through it to help keep the motor cooler. They put stickers on them because they offer more space to get the sponsors name in.

Those are the radiator shrouds. They help direct air into the radiator and keep some mud and/or dirt out.

The stock 250R's have them, except they are black plastic. The aftermarket aluminum ones stand out more and just plain look awesome!!