I want to use my own images and videos as background.
If a template has an editable background, there will be a ‘Background’ placeholder listed under Media. Depending on the template, you may use an image or a video as background.
Some templates have a solid color or gradient background by default, so you might need to change adjust options in the Customize tab of the editor sidebar in order for your image to display correctly. In order to do this change the background option from ‘Color’/’Gradient’ to ‘Original Colors’.
Transparent videos don't have a background (or rather it is transparent) so that they can be overlaid on top of other videos.