Incidentally bugger all in most cases I reckon unless you're working in trading. There's a bit of a vacuum at the moment so standards are low. Most orgs are willing to drag you through the relevant training.

In all my years of working in the financial sector (NOT trading I will add!), about as complex as it got was numeric precision, rounding, equation of a line and curve fitting which was all pretty easy. Oh and deciphering lots of poorly written regulation and rules.

Don't hit trading unless you like snorting coke and 100 hour weeks.

You can learn brush up on the terminology pretty quickly - the IBs will hire any solid developers who know their platform well enough (so java devs who appreciate core java + modern ecosystem; python devs who have experience with scipy/numpy etc. A big part though is willingness to jump in and remain for a while, to be able to tolerate the mindless processes and procedures that come with massive companies in a regulated industry.