There's not a huge difference between modular electronics and plain old electronics. When you dig into it a bit you still need to understand the same sort of stuff.

Practical Electronics for Inventors is a really good book to get started with this sort of stuff: https://www.amazon.co.uk/Practical-Electronics-Inventors-Fou...

Once you realise that under the modules there's the same MOSFET input stages, MOSFET output stages, diodes and voltages that you get in discrete electronics, it all becomes pretty clear how to make things talk and interact.