With locked scree, you can't. Yet, there are two other options you might want to try 1. use pop up view and drag in to bottom corner of screen so it won't come in middle if you want to use some other app. 2. open YouTube website in chrome, minimize chrome (go to home screen) now scroll down notification bar and press play button ▶️ and you will be able to hear but video will be running in background and you will be able to use screen for other work. If you find this helpful, please select 'accept solution' in this comment after pressing 3 dots.
