When the Amiga came, it was a mix of 3 components. Kickstart was like a BIOS on steroids with lots of functions above hardware (driver) level. Most of the DOS was commands in the c directory. Occasionally you had to start Workbench cos a game required it, which I guess was the GUI.
Kinda like the structure of Windows 95, except the "BIOS" was proprietary and luxurious, and when you were in Workbench programs usually used libs to do file stuff, while in Windows 95 pretty much everything was done with DOS interrupts, IIRC.
So what was later to be called "AmigaOS" is all those three. On PCs nowadays the BIOS is excluded from the OS, but on most brand computers the BIOS is and was part of the OS - in fact, early machines' OS was the ROM chip and nothing else.