Unity DOTS
Unity's data-oriented track running alongside GameObjects: an ECS, a job system and a custom compiler for dense, parallel game code.
The Data-Oriented Technology Stack was unveiled in 2018 as the answer to classic MonoBehaviour scripts not scaling to hundreds of thousands of objects. Three parts fit together: Entities, an ECS that packs components by archetype; a job system that spreads work across cores with built-in race detection; and Burst, an LLVM-based compiler that turns a restricted subset of C# into vectorised machine code.
The Megacity demo showed what it could do, but getting there took a while — Entities sat in preview with breaking changes for five years before 1.0 shipped in 2023. In practice most projects mix the two: ordinary GameObjects for most things, DOTS wherever there are a thousand of something.