The end screen is the page after the last lesson: your text (or a standard greeting) and a button back to the main page of the course.
To customize the course completion screen, open the course and go to the "End of Course Text" section in the Learning Process settings. Enter the text—a preview will immediately open the completion page. Students are directed here from the last lesson by clicking the "End of Course" button.
You can assign an auto-action to the end of a course: open the "Auto-actions" section, create an auto-action with the "Course" type, and select an event and action. A detailed breakdown of auto-actions is available in a separate section (cards or instructions will appear later); this is just the entry point.