You don’t need to build the engine to debug the engine source if you included the “Editor symbols for debugging” when downloading the engine. It’s a big download but easier than trying to compile the engine yourself. You can modify this option from the Epic Games Launcher in the dropdown for each engine version you have installed.
Once you have the debug symbols downloaded and your IDE configured you can launch using the Editor (DebugGame) configuration with the previously mentioned breakpoint set. This assumes that you are using a C++ project and not a pure Blueprint project.