I have a bunch of sonoff devices on my hubitat and only there, im not using their native app or hub, theres an option for firmware update which seems to do nothing. Im just curious if that ever works with devices and what the method the hubitat uses to retrieve the update is.
The vendor must provide Hubitat with a firmware file, or otherwise they must be released publicly. The "Update Firmware" command, which I assume you are talking about, matches some identifiers from the Zigbee data (so they only get applied to the correct device). There is some more information here:
I'm not aware of Hubitat having any Sonoff firmware updates (or any having ever been released, but I can't say I've paid attention). If there is anything, you'll see something in "Logs" when you try, as mentioned above.