Much of this is just “normal” Ubuntu admin. The trick is that you are modifying an image on a PC which was intended for arm64 architecture tools. You might investigate using QEMU for much of this. This URL might be of interest since QEMU is used to perform arm64 commands on the rootfs image of the host PC:
https://forums.developer.nvidia.com/t/tx2i-modify-rootfs-to-create-new-user-during-flashing/118076/5