In 6 your buildings in cities are categorised into districts. The districts take up a hex on the grid and receive bonuses based on adjacent hexes. A large part of the game revolves around planning your districts in every city as once they are placed, they cannot be moved. This is a slightly different playstyle compared to 5 where only the city location itself matters.

Some other changes were around science, policies and eras. You unlock policy cards which you can swap out for different bonuses when needed instead of a constant effect. Policies are just as important as science this time around, and researching science and policies is boosted by actions in the game instead of only using Great scientists/writers. Every set amount of turns the world enters a different era, which also offers different policy cards for that period.

There are no (or few) multiplicative bonuses. Having more cities is always beneficial.