What’s the difference between main.cpp and maincomponent.cpp?

How to adapt to these updates?

What updates do you mean?

If by main.cpp and maincomponent.cpp you mean the Projucer generated files for stand alone application projects, those have always been there. main.cpp contains the code to start up the application and to create the main window. You will rarely need to touch anything there. maincomponent.h/.cpp have the code for your actual custom content component. (That goes inside the main window.)