please access Jetson AGX Orin Series Design Guide for ethernet and uart chapter.
BTW, there’re five UARTs available. that’s UART1 on pin-8 for 40-pin expansion header.
You can make your own custom board for RGMII interface. The developer guide document also has gudiance for what is needed to change in device tree to bring up RGMII.
Thank you for your response. To clarify, I understand that the AGX module has an RGMII interface, but I was wondering if this interface is available on the Developer Kit?