Can someone please explain to an word not allowed caveman like me, in the simplest of terms, what the differences are between FLCH, V/S (actually, that one is self-explanatory) and VNAV?   These would be as they pertain to the (Fenix) A320 and the (iFly) 737MAX.      Just saw a RL video where a jet crew used FLCH on takeoff, something I didn't think was ever done.    Just trying to figure out how the RL pros use the modes.  

In general terms:

In a Boeing, FLCH (or Fight Level Change) will climb or descend the aircraft at the speed you have entered in the Autopilot Mode Control Panel.  VNAV (Vertical Navigation) will climb or descend based on the climb profile in the Flight Management Computer, levelling off for altitude constraints etc.  FLCH is less automated / requires more pilot paying attention, but allows greater control which can be useful in some scenarios.  VNAV is the most automated.

It works similarly in an Airbus, though generally, FLCH mode is called Selected Mode (either Open Climb or Open Descent), while VNAV is called Managed Mode.

I'm sure others can add more detail, but that's my simple way of looking at it.