Seatbelt over baby or car seat?

Anon because I’m embarrassed… We have a Joie I-gemm car seat that is usually on the isofix but we’ll need to attach it to the car with the seatbelt.I can see how to do it from the instructions but it’s not clear whether the seatbelt goes over the baby inside the chair,or just over the chair and the baby goes on top. I can’t seem to find the info anywhere to tell me! The seatbelt isn’t flush over the bottom of the chair as it’s pulled tight,so the baby isn’t flat in the chair,but putting the seatbelt over the baby doesn’t seem right either! Any (non-judgmental!😂) help would be appreciated please :)