The brake fluid reservoir sits on top of the brake master cylinder most of the time, but not always. I may have used them interchangeably.

 

Excuse my lack of knowledge in the engine compartment and what some of the parts actually do - are those two separate parts and do separate things?

 

The reservoir holds the fluid the master cylinder uses to push fluid through the lines and to the brake calipers. The fluid presses on the piston(s) in the caliper that press the pads against the rotor.

The kinetic energy of the pads rubbing the rotors is turned into friction and heat. The rotor is a heat sink and absorbs the heat, then dissipating it.

It’s a closed system, so the reservoir lowers only when fluid has leaked or ? Out of the system.

That’s my elementary understanding of simple brakes. ABS brakes are more complex.

A Google search will give a better understanding.