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.