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VST 2 was not so hard as you would imagine to get started. They are just a dll with a well defined interface. Sadly 2 is now deprecated by Steinberg but probably available. Don't know about VST 3.

However ... getting deep into DSP is another thing!

Check out the valhalla blog for DSP stuff. https://valhalladsp.com/blog/

Also, you might want to checkout CLAP rather than VST https://cleveraudio.org/

https://github.com/juce-framework/JUCE

Or even better: https://github.com/iPlug2/iPlug2. Similar to JUCE but with a permissive license.

Checkout the JUCE framework, it makes a lot of stuff easier