This post should be in the bus section btw, but to answer your question, the XEs mostly run on the M42 and M50, but they've also shown up on the M9, M57, M66, and M72, as well as the M11 school trips from 110th to 34th.
Quill tries to keep the XEs on the M42 and M50 since those two routes are close to the depot. They recently started running them on those other routes I mentioned towards the end of last year, but since 2020 started, they've gone back to only being on the M42 and M50.