If your theme has a Background video section, you have a few options for hosting background videos.
1. mp4 (recommended)
mp4s are reliable, load faster, work on mobile devices, and don't rely on third party services. These are the best option!
Go to your store's Files page (Shopify Admin > Settings > Files) and upload your image. Try to keep your file under 10MB if you can. You can copy the full URL of the mp4 file from this Files page.
The URL will look a bit like: https://cdn.shopify.com/s/files/1/2301/4669/files/your-video.mp4?v=1594049552
Upload your video to YouTube, and copy the URL of the video.
It will look like this: https://www.youtube.com/watch?v=Z0yhS_sJkMk
Upload your video to Vimeo, and copy the URL of the video.
It will look like this: https://vimeo.com/32918908
Copy and paste the video URL into the relevant setting in the theme. The video will play on loop behind the overlay text, with the sound muted (this is a requirement for autoplaying videos).
Please note that YouTube and Vimeo videos will often not autoplay on mobile devices - this is a limitation of those devices, and not something that can be worked around. When this happens, it will look like this:
Please always add a placeholder image, which will show if a video fails to play!