that jetson-nano-devkit.conf is board configuration files to flash the board, which include chip-id, emmc config, bct files, root file system size, all necessary binaries…etc.
you may download different L4T release versions via SDKManager, please also check JetPack Archive to understand the released versions of JetPack and L4T.
while you’d download the image via SDKManager, it will also save the release image locally to your host machine with JetPack version,
for example, ~/nvidia/nvidia_sdk/JetPack_4.2.2_Linux_GA_P3448/Linux_for_Tegra$
you may enable flash script to re-flash the board.
since Nano platform has emmc and sdcard versions, there’re different board configuration files you should used to flash Nano device.