What's the difference between a `ModuleNotFoundError` for Django core modules and one for custom apps when running `python manage.py runserver`?

A `ModuleNotFoundError` for Django core modules (like `django.core.management`) typically means Django itself is not installed or configured correctly in your environment. A `ModuleNotFoundError` for custom apps (`your_app_name`) usually indicates a problem with your project's specific configuration, such as an unregistered app in `INSTALLED_APPS` or an issue with the app's internal structure or location.