Well I'll give it a shot in layman's terms, I can't get to detailed because I work for a paint company(the best company out there

) and this is a public forum.
Refinish paint(general public can purchase)
hardener = isocyanates
Reducer = solvent
Hardener creates the chemical reaction(durability,hardness etc) with the resin system in which it is mixed, these hardeners & reducers are carefully formulated to feed off one another to give the best possible results.
Reducer lowers viscosity, over reduction creates less sag tolerance,under reduction creates bad appearance, orange peel,dry spray, near impossible spray conditions.
There is a reason every paint company makes a slow,normal,fast reducer/hardener combination.
Now OEM paint has a 1k & 2k system that is extremely more complex in the chemistry department. OEM coatings can be repaired without sanding a single panel, adhesion is based off a thermo reaction and are applied with a bell cup or recip gun for the best transfer efficiency.