In YouTube videos, when the user clicks it, the top-right 3D viewport shows the video footage as the background. When I do it. Nothing happens. What did I miss?
You have to looking though camera
I did (pressing num 0) but it did not work. It turned out that the camera entity’s viewport visibility must be turned on, otherwise the background does not appear. In my start-up file, I had disabled camera’s visibility because that yellow camera entity in the viewport often blocked the objects.
You can disable cameras in 3D viewport from Overlays menu in the top right of 3D viewport, that way I believe background will be kept, but you will no longer see wireframe of the camera