The Kickstart ROM contains the whole basic operating system of an Amiga. The 1.x release versions (versions 34.xx) of Kickstart were stuffed into 256 kB ROM chips (and into floppy disks for A1000). Release versions 2.xx (36.xx) and 3.xx (40.xx?) are 512 kB. Here is what is needed for a basic CLI window (and some information about which library takes care of what):

All that (excluding disk resident command software) is resident in the Kickstart ROM and all the ROM resident libraries, commands, devices, etc are always available. If you wish to use Workbench, you need LoadWB and two disk resident libraries, icon.library and info.library. (1.x release versions need them on disk. Newer Kickstarts might have those libraries ROM resident.)

Some words about the boot process: The AmigaDOS 1.x floppy disk bootblock contains only a few tens of bytes of program code (starts from offset $c) for checking whether dos.library is resident (ie in the memory ready to use) or not. If that library isn't resident, there apparently isn't any sort of complete Kickstart available and the bootblock program returns error code $ff. A1000 with its separate Kickstart disk is a possible reason why the code exists. I think the bootblock code also, when no error is returned, returns an address (dos.library's base address + $16) for the Exec to properly boot from the floppy. ("I think" because I'm not sure why the $16 offset is added to the base address.)