Your Garage - Lifters & Center Parking
Exploring Your Garage - Car Mechanic Simulator 2015
You can move any vehicle in your garage by walking up to it and pressing M. This will allow you to move it to another location. Only your own cars can be moved to and from the add-on parking garage.
In total you can have three client cars in the garage at one time, one on each lifter and one in the center of the garage. No, it does not work to stuff a car in the Paint Shop or Test Path to get more cars into your garage. Those still count as being in the garage.
You can however, move cars between the lifter, test path, and the center parking space. This can help you juggle the three cars that you can fit in your garage.