Tutorial question

In the tutorial build a midi synthesiser what is main content component refering to

You need to download the tutorial demo project to see the classes they’re referring to in the tutorial.

https://docs.juce.com/master/tutorial_synth_using_midi_input.html
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Download the demo project for this tutorial here: PIP | ZIP. 
Unzip the project and open the first header file in the Projucer.