Mostly it's manual TikZ in a text editor. For the book we use lhs2tex and some text inclusion tricks combined with parameterized TikZ diagrams to get a moderate amount of code reuse.

Some of our approach to the TikZ is written up here

stringdiagram.com/wp-content/u...

Thanks! I'd quite like to write such a textbook at some point. I need to finish my current book project first, which is the sequel to Introducing String Diagrams, jointly with Ralf Hinze.