What you say in the last 15 seconds determines what the viewer does next. Pick a goal and get a professional script.
Never end a video by saying "Thanks for watching, bye!". This is a signal for viewers to close the app. Instead, give them a specific next step. If you want them to watch another video, explain WHY that video is the perfect follow-up to the one they just finished.
Your audio CTA should match the visual elements on your YouTube end screen. If you point to the top left of the screen, make sure your "Watch Next" video element is placed there in the YouTube Studio.