Khan Academy is an excellent resource for learning about almost anything, but especially learning about math.
If you want to understand the functions that underlie oscillator construction, for example, learn about Trigonometry from them.
If you want to delve into the basics of computer science, they have you covered there too.
I did not expect to be doing a maths course when I stumbled upon an interesting looking synth in the App Store, but I have been inspired! I’m doing the Khan academy trigonometry. It’s amazing that these resources are available for free for everyone.
In addition to math, it would probably be worth looking at digital electronics classes if you need some pointers on the control signal processing side.
It’s probably a steeper learning curve, but if you have some basic electronics knowledge, some classes on electronic filters would provide some clarity around how filters work in a digital context compared to an analog one - this seems to me to be an area that prompts a lot of questions about filters.