Mainly time-series data processing through kdb+. There's an IDE and language tools for q (which is a thin layer over k) that is primarily written in q called: https://code.kx.com/developer/

Unfortunately it's closed source so I can't share much about the world of application and library development with q/k. If you want to learn more about how it's used though, I'd recommend checking out KX and kdb+.

I don't use it professionally or anything like that but most of the real world use as far as I know is in finance stuff.

This website has a few companies which use K: https://github.com/interregna/arraylanguage-companies