There are several different types of ADU
Detached ADUs are basically free-standing, newly-built structures in the backyard.
Attached ADUs are additions to the primary house. They are basically the same as a detached ADU, but they were constructed next to a house with shared wall or roof.
Garage converted ADUs, are existing garages that are converted for use. The garage may be attached to the house or detached.
Other internal conversions are existing non-livable spaces converted to ADU, such as basements, attics, hallways, boiler rooms, etc., mostly seen at multi-family houses.