First of, Bevy isn't just an ECS implementation. It's a game engine designed around ECS. While the ECS part is its core, Bevy also provides rendering, resource management, physics, etc. One of its other tasks, a rather important one too, is system scheduling: once you register systems, Bevy is responsible for dispatching them each frame, at the appropriate time, while also avoid data races over global resources and components.

Other ECS implementations might not, but Bevy does come with a system scheduler. You register systems (functions that operate over the components) and, through their parameters, Bevy's scheduler decides how to parallelize the registered systems while avoid data races.