Hi There, fbd38
I would suggest search for “OVER The Air Update for Nrf52840” or " BLE OTA" should show you what you need. I have read a couple of successful configurations and Yes the On-board QSPI holds the partition you setup prior too, with the size and code needed to DFU flash the Firmware from the Onboard Flash to the Internal. Resets and GO! (In one case the Original code was also preserved incase of a roll back)
You can ALSO put it into a Grove Expansion Board with the Extra Flash Chip installed. (see my Demo & Code posted)
HTH
GL 
PJ 
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