One possibility that hasn't been entertained much is the First Order could be the former Rebels (they won the war and got all the Imperial kit which they have continued to finesse).

And the Resistance are the remains of the Empire.

They were thoroughly defeated and have turned to using the tools of their conquerors (X-Wings etc).

Not saying this is what's happening at all just pushing it as another serving suggestion.

The Republic had trooper armour like the Imperials so the Alliance in displacing the Imperials they may have reverted back to controlling the Galaxy with white plastic ground troops.

Maybe the price of victory was to become the enemy.