Courtesy car or replacement vehicle? They are not the same thing
People use the two terms as if they mean the same thing. They don't.
A courtesy car usually comes from the garage repairing your vehicle, under your own policy. It tends to be small, it's often only for the repair period, and what you get depends on whatever they happen to have spare that week.
A replacement vehicle on a non-fault claim works differently. You've lost the use of your car through somebody else's mistake, so what you're given should reflect the car you've lost.
The difference matters most if you need something specific. A van for work, an automatic, seven seats, a towbar. A courtesy car rarely stretches to any of those. A properly arranged replacement should.