uf2 - https://github.com/Microsoft/uf2
UF2 is a file format, developed by Microsoft for PXT (also known as Microsoft MakeCode), that is particularly suitable for flashing microcontrollers over MSC (Mass Storage Class; aka removable flash drive).
Making your own UF2
To create your own UF2 DFU update image, simply use the Python conversion script on a .bin file or .hex file, specifying the family as 0xADA52840 (nRF52840) or 0x621E937A (nRF52833).
nRF52840
uf2conv.py firmware.hex -c -f 0xADA52840
How to compile and build
You should only continue if you are looking to develop bootloader for your own. You must have have a J-Link available to "unbrick" your device.
Prerequisites
Nordic's nRF5x Command Line Tools - nrf5x-sdk-dat