When you play a game it uses both.
When you're in-game you're going to be heavily using your RAM, as the data is dynamically being accessed and changed.
When the game is finished if you have a local save file that will live on the SSD.
Example:
If you're playing Doom, the save file and all game configuration information is living on the SSD. As soon as you RUN Doom and begin playing the SSD information is loaded into the RAM. While you're playing the game quite a lot of the game is living on the RAM, as it is being dynamically interacted with. When you save your game and quit the changes are permanently written to the SSD and the RAM isn't being used for the game any longer.