The JUCE library. Or is it like I meanwhile suspect it to be, that the JUCE modules are built together with the application?
The tutorial on Pojucer states, that it is needed to create projects and build files for different IDEs:
Learn how to install JUCE. Create an Projucer project, and generate Xcode/Visual Studio projects that you can run and debug.
What do you mean by a boutique frontend? And in following the only available tutorials, the Projucer is the only frontend in the dowload package.
I'm missing a document where the function of projucer, introjucer and the library sourcecode and how it is used as a (prebuilt) library is described.