Hi Syed — UX principles need to be served to and utilized by the whole team. But the UX designer is there to strategize, set, validate and facilitate the overall user-centered process through the tools and tactics you’ve listed so that the overall team can do user-centered work across the entire product. Some UX designers specialize in a handful of areas (i.e. I don’t do UI), but overall, we will have experience with all of them.
And yeah, it’d be great if UX design was a team — like a team of multiple UX designers to help pay attention to all the things for all products at all levels paired with all other teams. But that’s not my experience; I’m usually the only UX advocate in the room, and that’s exactly what I’m being paid to do.
In some companies, there is only 1–2 people in charge of doing the focus groups, business analysis, feature documentation, wireframing, IA and etc. This is generally the UX person(s). In others, especially enterprise scenarios, there is a lot of complexity, and non-UX specialists are now being taught UX principles to implement in their work or to confer with a UX specialist or member of the product team who specializes in UX.