From backups of the files. If you don't have backups, they are gone. You may see them in the recently deleted list, check the article here https

/blogs.systweak.com/2017/02/how-to-recover-permanently-deleted-photos-and-videos-from-iphone/
I think drphone has pretty good reviews for a recovery program you can try that
https
/drfone.wondershare.com/iphone-recovery/recover-deleted-video-from-iphone.html There are plenty of other file recovery programs but I think drphone has the best results from what I read before on other threads.