Lots of devices now offer new CODECs and compression methods for recording your videos on your new iDevices, the iPhone 12 and iPhone 13 are just a couple of new phones from Apple that default to using these high efficiency CODECs to achieve lower file sizes, more vibrant colours and High Dynamic Range video (HDR).
While these new storage methods (HEVC and H.265 for the technically minded) are great for users, they are not widely accepted on web services. Your device will often transcode into a more widely accepted format on the fly when uploading to instagram or Facebook, but that process is usually opaque to the end user.
Here's a few tips on working with compatible formats:
Set your device up to use Most Compatible Formats when saving your video. View a quick guide on the process on How to Geek.
Export video taken from a device in MP4/H.264 through third party software. This will re-encode your video into a format that is accepted by Sked and Instagram. Tools like Premier Pro, Quicktime and others can support exporting to supported formats.