Admittedly I ended up building a board with a microcontroller rather than a microprocessor because I could interface with the modem over spi too. However, this YouTube channel was great to work through:

http://www.youtube.com/@PhilsLab

It has some high speed stuff with ddr memory, fpga’s gigabit Ethernet and usb3 too so I’m pretty sure you could use the same concepts to build a board with a microprocessor on it. Proper grounding, emi, impedance control and length matching are well explained. I think he has some more advanced courses via his website too.

Additionally, this was an ah-ha moment video for me: https://www.youtube.com/live/ySuUZEjARPY?feature=shared