When Auto play is enabled in YouTube videos, the video moves onto the next one in the playlist. There'll be a small toggle in the top right corner of the video with a play/pause icon. If it's switched to the play icon, it means Auto play is enabled. So, to switch it off, tap onto the icon to ensure it's on 'pause' and this will stop the next video from automatically playing.
Oh I can imagine! It's always the little things that drive us mad! 😂