How much RAM do different back compat profiles use?

So I see a lot of conflicting information out there, and not a lot of specific information directly from Microsoft. What I'd like to know is how much RAM different BC profiles use, ultimately so I can have an idea of how big the RAM cache will be for a particular game.

So obviously I know the absolute base would be playing something on an Xbox One or One S. For everything running on that, the game would expect 5 GB from the system. Where it starts to get murkier is when thinking about an og/360 game with One X enhancements. I know they're definitely not using the full 9 GB a One X could offer, otherwise the One X version wouldn't run on a Series S. But I don't know if maybe it is at least using SOME amount more than the base 5 GB. (And I know the Series S is definitely running the One X version of 360 games, while also having at least some amount of a RAM cache, though obviously not as much as the Series X.)

The next level to that question would be whether or not the enhancements on the Series consoles also add to the RAM demands.