I asked this question of a dealer once.
For most places the double doors are usually wider and taller than standard, and can be unhinged if necessary. People just don’t usually notice that the door is bigger.
Other places with a lot of plate glass will usually have a window that’s not really glass and is plexiglass or plastic or something. This window is removable (or on hinges of some sort) and the car is pushed through over ramps.
My understanding is that this is a delicate and difficult process, so they only switch the cars in and out every two-three months or so. The guy also said that most dealerships don’t even drive the cars inside - they push so as to reduce the chance of accidents as much as possible. Walls and brand new cars are apparently are expensive to repair. Who would have known that?