Since iOS 10, Apple has provided the support for downloading HLS (m3u8) video for offline viewing.
My question is: Is it necessary that we can only download HLS when
The only way you can do this is to set up an HTTP server to serve the files locally after you've downloaded them.
The Live playlist uses a sliding-window. You need to periodically reload it after target-duration time and download only the new segments as they appear in the list (they will be removed at a later time).
Here are some related answers: Can IOS devices stream m3u8 segmented video from the local file system using html5 video and phonegap/cordova?