This may or may not be helpful, but Rob Papen has a very good book called "The 4 Element Synth". It lays out the basic building blocks of all synths and how they interact normally.
Additionally, you could get a demo or paid version of any of a range of software modular synths. U-He Bazille is interesting, for example, and isn't as hindered by latency and GUI issues as some others. If you spend some time with Bazille, open one instance with a preset patch, open a 2nd instance and re-create that preset patch, that will show you a LOT about how to make modular patches.
More generally, if you're not much experienced with synthesis, spending some time listening to and goofing around with software synths (demos or paid) is a good way to get some basic fluency.
Last, it should be emphasized, modular synthesis is by nature very exploratory.