Mainly divided into two aspects. First, if you want 16:9, set it to 16:9, and if you want 4:3, set it to 4:3. Note here that when exporting a video, it must correspond to the previous one.
The second point is very crucial, the mistakes that beginners love to make. Right-click the selected material in the preview window, select Resize to Screen, then right-click and select Keep Aspect Ratio, so there is no black edge.
I want to do all the above, but there is one link I can't understand.