Work with a professor from non-PhD-granting university in the US to earn PhD? I'm new on this website and am not sure how much of higher academia works.

Suppose I wanted to work with a professor in a particular field of mathematics at a research university (not necessarily a PhD-granting university) to possibly write a dissertation in that field. Must that university be a PhD-granting university? If not, how would I go about working in a particular field?

I'm currently working on a master's degree in mathematics. I have found that most of the researchers in the field I would like to pursue are employed at non-PhD-granting universities. Does that mean I won't get the chance to pursue research in the field?
AFAIK, almost every Ph.D. granting universities requires your supervisor to be employed at that university. The only workaround for your situation is to have your desired professors from other universities as your co-supervisor or co-advisor. Please note that this is something to be decided by your supervisor, not by you. Also, you need to convince a professor from another field to accept your not-related application in the first place.