A 6 year old doesn't have to ride in a regular car seat with the harness but a 6 year old likely needs to be in a booster seat if the seat belt doesn't fit them properly.
The booster seat is to help raise the child up high enough to allow them to fit properly with the seat belt so that the seat belt protects them.
If the child can fit in the regular seat of the vehicle and the seat belt fits the child properly then the 6 year old doesn't need a booster seat.
But if the 6 year old doesn't fit properly in the seat with the seat belt then the 6 year old would need a booster seat but a backless booster seat would be good enough for them.
When I was 6 years old I sat in the seat without a booster seat or car seat and used the regular seat belt but laws have changed since then and some states may require the 6 year old and even 8 year old ride in a booster seat.