[Q] What creates the "base" file system and layout (/boot, /system, etc.)? topic

Hi all
Any time I've opened an ADB shell ('adb shell') and listed the directories ('ls -l') I see directories like /boot, /sdcard, /system, /root, etc.
What creates that layout and the file system it lives on/in?

My best guess is that it created when "installing" a ROM in form of a zip file, using recovery (CWM, TWRP, etc.).

Best regards
Wuhtzu