Is the SD card slot only used for firmware upgrades? Can a spare SD card be stored in the controller or will the controller keep looking for upgrades because of card
present and cause malfunctions of the controller when flying? Can the SD card in the controller be used to download pictures and videos from drone?
Is there a place to find detailed answers to questions like this?

I have a 16gb sdcard (the one that came with the plus) always inserted, it has never been used except for the firmware update) nothing is saved on it but does not interfere with the operation of the controller

I always keep a SD card in the drone and not in the controller.

I use it to store a spare sdcard without loosing it and handy when the main sd goes wrong