DT is only for mainline kernels and it resists in /boot/dtb/ directory. There are usually more than one DT since the kernel is made for more boards at the same time. Which one has to be loaded is defined in u-boot. You can override this in armbianEnv.txt which is loaded by u-boot start script (boot.scr). If you by mistake started with the legacy kernel, there is no DT but Allwinner way of defining things.
We can only cover onboard hardware. Even that proves to be an extremely hard job. 3rd party hardware is on your own, but perhaps you could find people, who have done this, here.