hello user101803,
please check Product Design Guide, you should refer to [Jetson AGX Xavier UART Connections] for reference,
please looking for $ dmesg | gerp THS for the register uart ports, you should also check address mappings in the device tree,
i.e. public_sources/kernel_Src/hardware/nvidia/soc/t19x/kernel-dts/tegra194-soc/tegra194-soc-uart.dtsi