Nowadays, there is a lot of online education platforms to learn anything you want, free or paid access.
For C++, I can mention:
- SoloLearn (free)
- MITOpenCourseWare (free)
- Pluralsight (paid)
- LinkedIn Learning (formerly Lynda.com, paid)
- edX (free audit, paid for earning certificates)
- Coursera (free audit, paid for earning certificates)
- Platzi (paid, available in Spanish)
There is also Kadenze (free audit, paid for Premium access) for digital arts that also has a JUCE plugin development course from Output.
If you use any of it. Which one is your favourite platform? Why do you suggest it? Is there a trade-off between content and monthly fees?