ISOFIX
ISOFIX is the international standard for child-seat mounting: rigid anchor points built into the car’s seat, plus a top tether or support leg, that a compatible seat clicks straight onto. It’s far less error-prone than belted seats — misuse drops dramatically — and has been mandatory on new cars for years. Check a used car actually has the anchor points (usually marked with small tags) and that your chosen seat is approved for that specific car.
Example
Nearly every car on sale has ISOFIX on the two outer rear seats; the middle rear seat often misses out.