These plants were done on a stairway. (The plants are growing straight up. It's the picture that crooked...)
Murals can be indoors or outdoors.
This is a mural I have painted on a chicken house:
Entire rooms can be painted. Below are photos of an underwater bathroom I painted many decades ago.
I painted the murals below to make small, windowless rooms seem larger and more open. (Again, the artwork is straight. It's the photo that's crooked...)
Even the curtains and window sill are painted on a plain, flat wall.
This one was done on a life size thin canvas. Canvases this size are the biggest expense in doing a portable mural but they can go with you when you move. They also make unique gifts!