There are several models of Orin, so I don’t know which development kit you have, but you can filter by that. Then search for “trm” or “technical reference”.
Other documentation may be specific to the software installed. Mostly it is a release of Ubuntu, but with NVIDIA drivers added. With the drivers added it is known as “L4T”, and you can go to the documentation and other downloads for your specific L4T (see “head -n 1 /etc/nv_tegra_release” to know your L4T version) here: https://developer.nvidia.com/linux-tegra