I suppose it will be difficult to provide a precise response as it is a fairly vague question and the reality is quite diverse.
From my personal experience, the Quant I used to work with are using techno as R, Matlab combined with Visual Basic. Regarding more sophisticated tool coded in Java or C#, they are most of the cases inhouse frameworks.
So the only advice is to play with framework that allows probabilistic computation, and read a lot about Risk computation and Financial analysis.
As mgilbert said, Strata is a nice approach if you want to learn how an inhouse framework may look like. It seems quite complete and it is a good way to get familiar with the architecture of inhouse frameworks you may found in Asset Management companies.