I assume you mean ppl who literally have “mathematician” as a job title? A few I could think of…

• I’d guess most likely as an academic researcher. There are academics in just about any field you could imagine, a lot of which are even more abstract/“useless” than advanced math. Not a traditional “job” in the sense that academics don’t directly add value to the economy… but are paid to do research that hopefully other people can add value based on. Downside is that these job openings are insanely competitive especially for the aforementioned “less useful” fields, because they are based on an organization having spare money to support research…
• As a cybersecurity researcher maybe? A lot of modern-day cybersecurity (the original “crypto”, before it became associated with bitcoin) are based on advanced math, so I’d imagine such expertise is still needed
• Somewhere in finance maybe? A lot of modern-day finance are built on data science/statistics, although I suppose this job fits statisticians better…