Don't try to simplify it. You can't!
Grassland hills produces a base output of 1

. So if you place a city on grassland hills, it doesn't change the minimum output of 2

, 1

, 1

.
Plains hills with iron produces a base output of 3

. So if you place a city on such a tile, it does change the minimum output of
A plains river tile produces a base output of 1

, 1

, 1

. It doesn't change the minimum output of a city tile.
A plains river tile with silk produces a base output of 1

, 1

, 2

. It does change the minimum output of the commerce value of a city tile.
This doesn't mean it's a good idea to settle on resource tiles. Since, the improved value of a resource tile is hugely better than the base value, you typically still don't want to settle on a resource tile (although the above used example of silk just might be a resource where the improved output is not really worth it).