Your store, your rules, your theme’s playback
On Shopify nobody reviews your product video before it goes live. There is no marketplace policy deciding which slot it gets and no six-image rule. The limits in Shopify’s Help Center are generous: uploaded videos up to 10 minutes, up to 1 GB, up to 4K, as .mp4, .mov or .webm. Shopify converts .mov and .webm and serves them as .mp4 or HLS, streaming at 480p, 720p or 1080p depending on the shopper’s connection.
What actually decides how your video behaves is the theme. Shopify says its Online Store 2.0 themes and the Horizon family support video; for anything else, check with the developer. The product media article doesn’t state a per-product video limit, a required aspect ratio or how autoplay works, so the only reliable test is your own product page, on a phone, after upload.