Irl they become actuaries. It’s usually applied mathematics and if by mathematician one means someone exploring mathematics alone then this is accurate. CompSci kinda falls half into this and half outside of it.

It reminds me of the statement by Clifford Stoll, (paraphrased) the first time you do something it’s science, the second time you’re an engineer, the third time you’re a technician. link.

In essence most statistician work ranges in the engineer to technician level of mathematics.