Series S is intriguing because Flight Simulator is a beast on PC, so the concept is getting it to run and look good on a four teraflop GPU paired with only 8GB of accessible memory is an extreme challenge. Resolution is therefore cut to 1080p, with no kind of reconstruction. It's native 1080p, with 1080p UI but just like Series X, there is no sign of dynamic resolution scaling. However, there is the sense that Series S is compromised in other respects beyond the pixel count and level of detail is the main casualty - draw distance is pulled in (to my eye, more equivalent to PC's medium setting) and while terrain generally looks fine, it's the cities that suffer most. Not only is the draw distance pared back, closer detail seems to show lower fidelity models streaming in. It still looks good judged by its own terms, but clearly there was some hard work needed here to get this to work.
There's an interesting balance between Series consoles then: the X leans into image reconstruction at the expense of core resolution, freeing up GPU resources to double down on detail. For Series S, detail is pared back (perhaps necessary bearing in mind the memory constraints) but it looks like a full 1080p presentation. In my view, Asobo has made wise choices to get the best Flight Simulator experience out of each of the machines - but this does lead to an interesting disparity that actually favours the junior Xbox. To put it simply, Flight Simulator runs more smoothly on Xbox Series S.