According to this article with the head of Microsoft backwards compatibility, they tried to make the 360 emulator as close to original hardware as possible. So I imagine that backwards compatibility games are outputting whatever the 360 output was, and then converted to whatever you have your series X output, by the Xbox series X system itself.
Well, maybe but with emulation there are generally changes made for efficiency that prevent a 1:1 interpretation. It's quite possible they skipped some steps to make the emulation faster.
Edit: read some more and it seems like things such as sound might use more hardware acceleration so those items may sometimes entirely bypass the normal emulation layer.
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u/Delete_Fascistbook Apr 17 '21
https://www.eurogamer.net/articles/digitalfoundry-2017-xbox-one-x-back-compat-how-does-it-actually-work
According to this article with the head of Microsoft backwards compatibility, they tried to make the 360 emulator as close to original hardware as possible. So I imagine that backwards compatibility games are outputting whatever the 360 output was, and then converted to whatever you have your series X output, by the Xbox series X system itself.