Audio subsystems (wwise, fmod).

Physics subsystems (havok, ISI).

Procedural systems (Gaea, Houdini)

Vegetation (Speedtree)

VFX subsystems (Nvidia Gameworks)

First party SDKs (Sony Playstation, Microsoft NDK, Horizon/Quest).

Pathfinding (Kytheria, Mercuna)

Cutscenes/Videos (Bink)

UI (Rive, Neosis)

Networking (Photon, Coherence)

Theres… thousands more, if you’d like me to continue.

A very large percentage of multiplayer games keep the backend in an MS SQL or Oracle cluster.

Web servers, message brokers, physics engines, anti cheat, fraud detection, flood mitigation, ranking systems, chat moderation, match making systems. There are thousands of possible components which may have been licensed in any given game server system. In some cases the entire game engine runs on the server.