Dexed, Odin2, VCV Rack, Audacity, Obxd, Helm and Surge are all open source and you can find their code on GitHub. JUCE seems to be the framework used to make most of the above compile across platforms.
VCV Rack does a good job at modelling analog synthesis, Dexed does a great job of modelling FM synthesis (this project is based off a 20% project at Google). It might be worth spending a week compiling the above, finding bugs and submitting pull requests to the projects. If this is the type of work you're looking for then doing it in a public fashion for free for a short period of time should help in your quest.
Audacity has a very large code base and there was a good CppConf talk on it a few years ago where one maintainer attempted to try and plot out its complexities. Many of the other code bases are reasonably concise and digestible.
Songs will often have 10s and in some cases 100s of instances of VSTs being used so there is a huge focus on using CPU cycles as efficiently as possible and moving GUI code off to the GPU to save as many cycles as possible. Code stability is also extremely important, treat your code like it's running a nuclear power plant.
It seems C++ rules the waves in the audio world from all code I've examined. I've dug through LinkedIn and it appears few audio tool manufacturers employ more than 100 staff. The revenue distribution among firms is a lot more spread out than you'd find in other tech industries as well. Boutique firms and even one-person shops seem to do well. I'm not sure if there are any real gatekeepers to this industry beyond your ability to market to cost-conscious creative types.