unrealengine.com/learn
Unknown
Unreal Engine Learn is Epic Gamesā official learning portal offering free tutorials, guided courses, and hands-on projects to master Unreal Engine, including multiplayer networking, blueprints, and C++.
More resources on Multiplayer Game Development
Unity Netcode for GameObjects Documentation
Official free docs and samples for Unity's multiplayer solution
Client-Server Game Architecture
Gabriel Gambetta's authoritative client-server guide, frequently cited
Multiplayer Game Programming
Multiplayer Game Programming - A book resource
gamedev.net
GameDev.net is a comprehensive community hub for game developers, offering tutorials, articles, news, forums, and project showcases on programming, art, design, and audio.
photonengine.com
Photon Engine provides a cloud-based real-time multiplayer networking platform for games, with SDKs and tools for popular engines to build, host, and scale multiplayer experiences. The site offers product pages (Realtime, Fusion, Quantum), extensive documentation, tutorials, sample projects, demos, pricing, and other developer resources.
Developing Online Games
Developing Online Games - A book resource