Some personal observations:
It appears that the LOD & Mipmaps in Microsoft Flight Simulator scale pretty similarly to Google Earth. Although at high altitudes & over long distances, the Mipmaps used by Google Earth are extremely low poly & of low quality. Maybe Bing 3D does it better, I don’t know, because I didn’t use Bing Maps. I found their app super janky & inconvenient. I’m more comfortable with Google Earth. Regardless, it appears that Microsoft is running some algorithm on top to clean out the low poly buildings, trees, etc. at high altitudes and over longer distances, to make them appear much better and give the impression of better LOD. Of course this is just speculation. Maybe the LOD is actually spectacularly higher, but my opinion is that Azure is doing that for them, thus saving on computation costs. Nonetheless, it’s still incredible.
Areas that are not covered under 3D satellite imagery data, The Sim seems to be using good old autogen system, although vastly improved over ESP platforms and seen to blend in well with the rest of the world.
FSX vs FS2020 - Screenshot comparison: https://www.avsim.com/forums/topic/556466-screenshot-comparison-fsx-vs-fs2020/
Scenery Locations: https://www.avsim.com/forums/topic/555678-e3-trailer-scenery-locations/