Welcome to the Shopify community!
and Thanks for your Good question.
Please share your site URL,
So I will check and provide a solution here.
Eunice here from the Shopify team, thanks for posting, and welcome to the Community!
From your message, I'm not sure if you're looking to upload a video to your password page, or to your homepage. To cover all basis I've included both in my answer below:
Adding a video to your password page
If you want to make customizations to your password page that aren't available in the theme settings, such as adding a video, then you can edit the theme code. Shopify has a guide for this here, but, it doesn't show you how to add a video specifically. In this case, I recommend reaching out to a Shopify Expert for assistance or waiting for further responses to your post.
Adding a video to your homepage
The following themes, all available in the Shopify Theme Store, come equipped with the ability to use a video as a background and the ability to show a video on the home page:
I hope this helps! If you have any questions don't hesitate to sing out here.