The two most important resources when developing with JUCE are the official JUCE documentation & the JUCE forum.
The API documentation for JUCE is very good in my opinion. All of the classes & functions are clearly documented.
The same goes for the forum, every time I asked a question, it was answered on the same day, which compared to other communities is simply awesome.
If you like to learn using videos, you should be able to go from beginner to advanced using the resources found mainly on YouTube.
The JUCE team has their own YouTube channel, where yo can find all of the talks from the yearly Audio Developer Confrence (ADC). The content goes from general audio development to JUCE basics to advanced, companies showing of their workflow using JUCE and much more. I will link to specific talks in later sections.
The Audio Programmer is a YouTube channel which almost only does JUCE related development tutorials. He has videos for the basics on getting up & running with the library, introduction to Components, an introduction to audio plug-in development and interviews with people working in the audio developer industry. Each video is ~20-60min long.
If you are new to C++ and audio this is a create place to start.
If you are already pretty good in the language and have a basic understanding of audio in the digital world these videos will probably be a little to slow for you.
The Audio Programmer also runs a Discord channel for everything related to audio development. I hang around sometimes as well.