Im confused.
I connect my tablet via WiFi to my RC, which is connected to the Spark.

At this point, my phone has no internet (‘cause WiFi is tied to the RC).

So, where does the firmware check and download come in? Does the app know what device I have and has downloaded firmware in advance ready to push when I connect?

You can also use a PC with DJI Assistant 2 -

Download from -https://www.dji.com/spark/info#downloads


Here you go -

Also watch -

Hello. We apologize for the confusion that may have caused you. It is really normal not to have an internet connection when your mobile device is already connected to your RC. You can check firmware using your DJI Go 4 app or DJI Assistant 2. There will be a prompt on your screen in DJI Go 4 app if the firmware update is required. If upgrading the firmware using DJI Go 4 app failed, you can use the DJI assistant 2. You can check if you are in the latest firmware using DJI Go 4 app. Just go to the camera interface > General Settings > About. DJI Go 4 app has an option to select the model of your drone. You can also check these tutorial videos:

Linking the Remote Controller:
Updating the Firmware with DJI GO 4:   
Updating the Firmware with DJI Assistant 2: