A survey of systems theory approaches and applications

I've been meaning to look into various general theories about systems and processes, but the field seems pretty obscure and ill-defined. Category theory seems to have been popping up in relation to this since the 70s, but I don't know if this stuff has been successfully applied to modelling any real-world phenomena. The late Robin Milner was working on some sort of process formalism stuff, but what I tried to read of that was extremely formalism-heavy and very light on the motivation. Baez's Rosetta paper tries to unify physical processes and computations with a category theoretical formalism.

One basic theme seems to be looking for a formalism that deals with processes instead of static objects. Process philosophy sounds like it should be relevant.

It seems obvious that better tools for understanding complex processes would be nice, but given that systems theory has been a thing since at least the mid-20th century and seems to remain pretty obscure and confusing despite people having struggled with plenty of complex systems in between, it looks like it might not be a terribly handy or powerful tool.