regarding the circular motion. If there is a mass that is placed on top of a car, and there is a static friction force between these two surfaces, and the car is moving in an arc shape with a certain radius, will fc=mv^2/r be the acting static friction force between the mass and the top of the car? Or will it just be the normal force, since we are neglecting the friction force between the wheels and the road?"

regarding the circular motion. if there is a mass that is placed on top of a car, and there is a static friction force between these two surfaces, and the car is moving in an arc shape with a certain radius, will fc=mv^2/r be the acting static friction force between the mass and the top of the car? or will it just be the normal force, since we are neglecting the friction force between the wheels and the road?"

The static friction force between the mass and the roof of the car is mv^2/r, as you said. If it was something different, the mass would slide. If mv^2/r exceeds the ability of the mass to reist sliding - that is, if it exceed \mu mg - then it will slide off the roof.

The friction force between the wheels and the road is also Mv^2/r, where here M is the mass of the car plus cargo.