Jetson modules have a lot of pins, and many of those pins can be programmed for different functionality. Those modules mount to a carrier board, and if a manufacturer wants to, they can change pin function to better suit the layout they want. Two carrier boards could for example have exactly the same functions and connectors, but one might swap two pins of the module and reprogram them to also be swapped in the module compared to the other. The drivers would be the same, but swapping one module for the other on the different carrier boards would result in those functions failing or behaving oddly. The device tree is how those functions are set up and how the pins know what arrangement of options to use (a.k.a., “firmware”).
Third party manufacturers normally supply their own board support package (BSP, the flash software). If the manufacturer chose the exact same layout as the NVIDIA dev kit, then they might just tell you to use the NVIDIA dev kit flash softaware. Otherwise the manufacturer might provide either of:
A patch to apply to the NVIDIA software, basically a device tre.
A whole new set of flash software, which in turn is mostly the same as the dev kit flash software, but with device tree (and perhaps branding) edited.