I would like to recommend a course on N-Tier Architecture and advanced networking using base UDP networking libraries such as ENet. This library lets you add reliability function to UDP rather than being stuck with TCP such as with Mirror. TCP ultimately limits the scalability of the game.
Applicable to to MMOs, Large Community FPS/RTS, etc.
There is a HUGE gap in learning materials for networking for games other than using stacks that exist but are limited, such as Mirror. We really need to fill that gap with learning how to code the network stack.