If I power up the drone first, then the controller, it loads it automatically. If I power the controller first, then the window pops up with the different possible drones you can run with it and you choose which one you want to load. At least...
I use my RC2 to control either my Mini 4 Pro, or my Neo. The controller asks on bootup to choose which one, and then it loads the appropriate firmware.
I'm sure this will be here on the site already somewhere, but my searches didn't find it, so copying here. Much more detail on those battery error code lights than in the manual !
(Graphic courtesy of Drone DJ)