One of the best: https://gafferongames.com/

shameless plug: https://github.com/0xFA11/GameNetworkingResources

Part 1: https://www.youtube.com/watch?v=W3aieHjyNvw

Part 2: https://www.youtube.com/watch?v=odSBJ49rzDo

The main difference in modern games is the set of things in the world that might need sync has grown, much more than the available bandwidth.

So need sophisticated logic to filter what needs to be updated in a given frame based on relevance, to fit within a packet,

And of course now you need cloud systems for matchmaking, telemetry, etc.