What are the functions and differences of Android's Download Manager and Downloads native apps?

I would like to know **what are the functions and differences of the Download Manager and the Downloads native apps of Android OS**? For example, I have downloaded and set Firefox as my default browser, and in it's settings, I see the option to use external Download Manager; so I wonder, is this app dependent (i.e., subordinate) to the native Download Manager or Download app? Or is it's **own** downloading utility independent from native downloading apps?