Fuchsia's Garnet layer does not provide a full-fledged end-user UI. Instead, it provides services that provide a foundation upon which to build secure, performant, multi-process UIs.
These services include:
Scenic (doc) provides a retained-mode scene graph that allows graphical objects from multiple processes to be composed and rendered within a unified lighting environment.
The input subsystem (doc) is responsible for discovering the available input devices, and allowing clients to register for events from these devices.