The same applies to the large screen in my school's auditorium. The central screen is designed to fit the size of the stage wall, making the screen and the entire stage look quite symmetrical. However, using the screen alone doesn't match the aspect ratio; that large screen is a massive 2.4:1 widescreen. When using that screen, we usually prepare all the videos and PPTs we need in advance according to the 2.4:1 ratio. If other teachers use PPTs with the wrong aspect ratio, you can stretch them as the questioner suggested, or leave black borders on both sides, but black borders on both sides look a bit ugly. Basically, everyone just creates 2.4:1 PPTs or stretches the large screen to fill the screen.