Yes, of course the Component class is defined in juce_Component.h, but why are you posting here to ask that, rather than just looking in the file yourself?
Use your IDE:
In XCode simply hold CMD and click the “Component” text and it will show you the definition.
In VisualStudio right-click “Component” and select “go to Declaration” (or Definition? not sure if they distinguish there, I am not often on Windows).
On Linux there are too many options, but most of them have a similar feature too…