I rent a garage in a building with a door controlled by a radio remote. I wanted to be able to open the door from my phone. I first started by looking at the remote and the receiver. I wanted to copy the signal from the remote but I figured out that it was a rolling code system so it was not possible that easy.
I decided to keep the original emitter and to control it with a Raspberry. The Raspberry would receive the instruction from my phone and would trigger the emitter through an optocoupler.
This is the circuit I ended up with.
I then added a Siri shortcut so that I can now open the door without even touching my phone.