usually there's a seat belt locking mechanism or the car seat itself has lockoffs to lock the belt in place. the belt isn't freely retracting and extending.
latch/isofix has limitations which is why it's not the de facto install mechanism across the board. there's a 65lb weight limit on most anchors (including the car seat - which can be 30+lbs by itself), and most sedans don't have a set in the middle rear seating position but a lot of people will incorrectly install it that way.
it is okay for a car seat to have a little give. they're designed not to be installed so tight it damages your car in the process. rear facing car seats will sometimes flip into the seat back and 'cocoon' in more severe accidents, and that's by design.
latch/isofix has limitations which is why it's not the de facto install mechanism across the board. there's a 65lb weight limit on most anchors (including the car seat - which can be 30+lbs by itself), and most sedans don't have a set in the middle rear seating position but a lot of people will incorrectly install it that way.
it is okay for a car seat to have a little give. they're designed not to be installed so tight it damages your car in the process. rear facing car seats will sometimes flip into the seat back and 'cocoon' in more severe accidents, and that's by design.