We have a pet door in the door out to the screen porch so the cats can go in and out without letting the heat or cold in.
It's just a simple kind. I think we mounted it a little too high. We thought they'd probably like to have it normal walking height but in practice they seem to prefer to crouch a bit. They got used to it pretty quickly but they still have to think for a while before going through, even after about 5 years.
The cats really like that it has a clear window in the flap itself (even though it is installed in a clear door?!). They always peer through carefully to see what's on the other side before going through and when I clean that part off well they go through more quickly and happily.
I'm not sure the brand or model, but it has a plastic frame, a swinging plastic flap that has a magnetic bit on the bottom to keep it closed generally but allow cat access. I think it has a way to lock it too but we don't use that.