Manufacturer’s own car seats are available. Some manufacturers use their own seat which is only then specific for that particular brand of vehicle. If it is a multi-fit car seat recommended by the car manufacturer, then it can be used in other vehicles with no great problems.
When it comes to booster seats, there are some vehicles that have them built into the back of the seat which pops up for use when travel with children. Although after fit seats do tend to give far better side impact protection so the inbuilt pop up seats should only really be used as a temporary measure if necessary.