(Volvo) Sit-up – How fastened in the car?

Hello, the upcoming car change is the reason for reorganizing the child’s seat situation. My big one is now ready for a seat increase, whereby I have the model of Volvo with backrest in mind: https://www.volvocars.com/en/v/buy/accessories/child-seats Some here have determined the model (or similar). Therefore my question: How is the part fixed in the car? Isofix doesn’t seem to have it. If the child is sitting on it and strapped on it, no problem. But what if the child is not on board? Then the part prunes through the area, if I don’t buckle it up, right? And every time the seat is strapped, it’s annoying in everyday life…