Writing theory section of thesis - feels like I am just copying
Although I'm not in "physics/applied-math", but, rather, "math", I see the same or similar issue arise. Let's consider the point that, by this year, the "context/background/set-up" has very likely been highly optimized, both for effectiveness and for succinctness. Nothing wasted, nothing superfluous, etc. So it is likely to be hard to "improvise" much on its description without making it worse in some way. This may be the case even down to small notational conventions, wording, etc.

So, be absolutely scrupulous in citing, and try to have it all in your own head so that you can write it yourself (not literally copying), even while citing, ... and remarking something like "we recall some standard facts/ideas/set-up, e.g., from [X]."