Fuchsia has this reference documentation:
FIDL (or "Fuchsia Interface Definition Language") is the IPC system for Fuchsia.
Tools for Fuchsia source contributors.
System calls allow you to make requests from the user space into the Zircon kernel.
Zircon is an object-based kernel. User mode code almost exclusively interacts with OS resources via object handles.
The Zircon kernel receives a textual commandline from the bootloader, which can be used to alter some behaviours of the system.