1) how does inertia affect a person who is not wearing a seatbelt during a collision?

Inertia is the tendency of an object to stay in motion when it is in motion. This is basically Newton's first law. The person in the car is still in motion when the car crashes and If there's no seatbelt to increase the time of the crash then there will be a larger force.

But the inertia of the passenger pushes the passenger forward(relative to the car) because the passenger still wants to go forward due to inertia. The seat belt applies contact force on the body of the passenger thereby preventing the passenger from colliding and preventing any injury.