If you want to know how the features of these programs are implemented (I'm talking about filtering, synthesis, mixing, etc.), read as much as you can about digital signal processing (DSP). If you are able to take a DSP class at a college or university, you are likely to get a lot of good information from this. (I myself took a couple of such lessons, and they were fantastic). In addition, there are dozens of books and research papers on various aspects of DSP; just try to find a good one (or a few) and dive. A Google search can help you get started.
If you want to know more about the audio program as a whole, that is, how to access the audio resources of the operating system (microphone / speakers / etc.), I think I could repeat Dan’s suggestion to look at Audacity ...