How to add video thumbnails to product

There are several ways for this.

If you use Content Egg in product, you can use Youtube module. Theme will create automatically thumbnails and video section in all Content Egg Product layouts

But also you can add video section if you place links from youtube or vimeo inside Video panel under Image gallery panel (right side of Admin area for products)

Place each links to video from new line

If you use Frontend Form for Product submit, you can also allow users to fill this field and attach videos to their products. For this, create Textarea in Form and use key of meta as “rh_product_video”