All sorts of services, databases and their integration with each other. Most often it is the infrastructure of organizations, all sorts of CRMs, 1Cy, and more. Unlovedly creative programmers for the fact that this is b2b, which cannot be boasted to friends, and b-programmers are cursing "galley" programmers, that this work is not creative, routine, and the performers are stupid and unskilled, who cannot be bearded start-ups. Well, since these systems do not attract customers, tons of Legacy code nest there.
It can be said that depends on the language. Overwhelmingly, this is Java and .NET.