Yeah, there's a ton to learn! Another good resource I just thought of is Omri Cohen. He mostly uses VCV Rack which is a free software version of Eurorack. I'd highly recommend watching some of his videos and patching along with him in the software. But yes, VCAs are useful for many things, but their main use is controlling volume of a signal. The fun part in modular is that the signal doesn't need to be audio. You can send an LFO or an envelope or some other form of slower moving voltage and control it with another voltage source so that it has more and less of an effect on whatever parameter you want it to control! And a cascading VCA also works as a mixer. So if you use an output from channel one, just that signal comes out of the output, but if you have inputs on channels 1-4 and only channel 4 output, all the signals come out of one cable!
The standard for cables is TS/mono cables, although the headphone output of that Behringer interface is likely TRS/stereo. The cv/gate outs on the akai are likely TS as well, but I think the audio in is TRS. So I'd recommend a TRS 1/8" to 1/4" cable going from the Behringer headphone out to the Akai audio in











