[bees_buzz]

There are a few ways.

Maybe a research group wants to investigate a connection to another discipline.

Maybe the research group comes across a talented student and sees the potential for a research grant (how this works depends on the country).

Maybe the student has the means to self-fund and so the university happily collects a pay-cheque.

I fell into the first category - I did my PhD in a quantum information group despite my background being in mathematical logic. I was accepted because I knew a lot about different formal logic systems, and they wanted to study connections between those and the formalism they were using for quantum information.