I had one in my old house. It was on the slider door, the stationary door was moved over on foot from the frame, the doggy door had a panel that went in between the stationary panel and the frame, with the doggy door on the bottom. My dogs learned how to use it in less then 1/2 hour. We also had a 6 foot block wall fence around the yard, so no problems with any other animals. I wreally liked it, I never had to worry about getting up to let them out. Now I don't have one and it is a PITA to get up at all hours of the night with them. I know that;s my fault, they learned that by having the door for 9 years.
I have seen them installed in walls of peoples homes, too.