Is there any justification given as to why plains which have no resources or trees in them grant production other than "it was the way the game devs wanted it?". I can't think of any rational reason for it, they should just be lower quality grasslands.
That's just highly simplified & stems from civ 1. In civ 1 you basically had:
(1) Flat area with lots of water & good soil = grasslands, food, occasionally also production (in civ 1)
(2) Flat area with less water/soil = plains, less useful for farming, but maybe more useful for mining/infrastructure
(3) Hills = Hills, less useful for farming (In civ 1, in civ 6 these are superduper all around tiles), but great for mining, especially if there is coal or iron to be found
(4) Forests = Forest, a little food & production
(5) Jungle = Jungle, food
(6) Desert, tundra, ice = less useful
(7) Mountains = Mountains, only production
-> You see that in civ 1 also grasslands could yield small amounts of production, which were the best tiles in that incarnation of civilization.