I used the JUCE framework, which is free for personal/GPL usage (or you can pay for a closed source license) and gives you cross platform. The examples are pretty good for getting the basics, there is also The Audio Programmer YouTube channel that has a lot of content (the only bad thing I would say about it is that some of the early videos sometimes use bad concepts as it seems this was a sort of a documentary series for learning plugin programming for the channel owner). The JUCE community forum is really friendly and helpful (and there is also a discord channel for TAP). Otherwise search engines were my main source to learn (mostly to refresh my C++ knowledge), and I managed to land myself a day job programming a plugin that helped a lot just by looking at the existing code, but that obviously a very rare occurrence!