Not much to say that Julius doesn't... open course materials for (almost) everything you might need in audio processing.
If you want to peak into some source code - you can look into Rubberband library:
https://breakfastquay.com/rubberband/
Rubberband is one of the time stretching/pitch shifting algorithms used in Reaper. You can download reaper trial and listen to the results with different parameters to see how you can tweak the code and if that gets any results you're happy with:
MTG Barcelona has been doing R&D for Yamaha since the 90s. They have published a lot of work on time-frequency transformation and have certainly implemented harmonizers and time stretchers. Look for papers and thesis by Jordi Bonada, Alex Loscos and certainly others too: https://www.upf.edu/web/mtg/research/publications
Needless to say pitch shifting is nothing new, so going back to research publications from the 90s may help. Publications might be found in early conferences of DAFX, ICMC, IEEE Mohonk, ACM multimedia, JAES, etc etc. Try keywords like "waveform similarity overlap add" WSOLA, "Lent's algorithm".
The musicdsp mailing list has discussed pitch shifting many times over the years. Participants have included engineers with fairly intimate familiarity with algorithms employed by Eventide, z-plane, etc. I would search the archives (you may need to do some digging to find all of the archives dating back to the late 90s).
Maybe look at expired patents from Creative Labs, Eventide and Antares if you feel comfortable exposing yourself to that literature.
I often recommend also Music and Computers originally out of Columbia. [1]
Also [2] is a decent book for overall dsp concepts.
[1] DAFX - Digital Audio Effects (Second Edition) Edited by Udo Zölzer https://dafx.de/DAFX_Book_Page_2nd_edition/index.html
[2] Understanding Digital Signal Processing, Richard Lyons
C++ for Real-Time Audio Programming: https://learn.bela.io/tutorials/c-plus-plus-for-real-time-au...
A blog post about an open source C++ pitch shifting library: https://signalsmith-audio.co.uk/writing/2023/stretch-design/
And accompanying ADC talk: https://www.youtube.com/watch?v=fJUmmcGKZMI
Back in university, I heard lectures on FFT and its applications to audio signal processing. So open access university courses would be the second place I'd look. The approach I always try first is to ask people I know if they can recommend a conference/meetup. For example, the annual JUCE events appear to be chock full with VST plugin developers. There's also private schools like SAE where you (or your employer) can pay for you to have an hour with one of their lecturers to ask questions.
Audio programming is a lot of fun but it's the most challenging domain I've ever worked in. You have to be very careful with what you do on the audio thread. No locks, no memory allocation etc. Messing this up can result in some really ugly audio artifacts.
Not sure if it's useful. It's probably going to involve granular synthesis.
Why?
Not questioning your motivations.
Rather I’m curious what they are.
For example, many of them don’t have great low end. Some are “sluggish” and need external enveloping. Getting a sense for what’s out there can help to provide a North Star when you write your own. Some classics are the Eventide H3000, IZotope Vocal Synth, TC Voice Live, Antares Harmony Engine, and Soundtoys Little Alterboy.