A video that plays in Chrome on your laptop and shows a blank frame on an iPhone is almost always
Safari on iPhone plays H.264 video in an MP4 container reliably. WebM and un-transcoded files from other sources are the most common codec cause of a blank player.
Autoplay on iOS needs muted, playsinline and autoplay together. Any one missing and the video sits on its first frame waiting for a tap.
iOS Low Power Mode and the Auto-Play Video Previews accessibility setting both block autoplay on the device, and no code fixes them.
Shopify-hosted video is transcoded for Safari automatically. Self-hosted files from Files or a third-party server need the codec, MIME type and byte-range support checked by hand.
Usually codec or container. Chrome on Android plays WebM and a wider set of MP4 profiles than Safari does.
The full guide has the formats, the deadlines and the worked examples.