In my case I am adding the ability to control my Garage Door Opener (GDO) to my 2005 car that does not have this built in.

I had already upgraded my radio to an Android head unit and it has three USB cables out the rear of the unit.
I am going to attach an Anrduino Uno with a Relay Shield to one of the USB cables.

I took a spare GDO remote, opened it up and soldered a couple of wires where the contacts of the main button are, such that when I connect these to one of the relays on the the Relay Shield it can effectively 'press the button' and open / close the garage door (via the RF signal from the remote).
I then removed the 'button' battery from the remote, and since it was a 3.3V battery I just soldered two wires to the + and - contacts on the remote and wire these up to the +3.3V and GND on the Arduino.

So, when the car (actually a truck) turn on the radio turns on, powering up the USB, which powers up the Arduino and the GDO Remote.

I was trying to develop an App for my Phone (first) and then transfer it over to the Head unit, to control the Arduino (via USB) to open/close the appropriate relay on the Relay Shield using the Android IDE but this looks much easier to use RemoteXY for this.

I just need a simple screen (horizontal) with a title (Garage Door Opener) and a big button in the middle (Door Open / Close) that every time the button is pressed and released it closes the Relay for 1/2 second and then  opens the relay.
Which is perfect for RemoteXY in the USB mode and I test the App with my phone before I install it in the truck (just a different cable as my phone is USB-C and the truck is USB-A).