Animated lock screens are a fun gimmick. Your phone comes with a few default options. But if you want to create your own on Android or iOS, you’ll need a third-party app.
Rotate your phone screen to landscape | no copyright video | free download link
On iOS, you can use intoLive to turn any video, animated GIF, or photo set into a “Live Photo.” (You can also use any “Live Photo” you’ve taken with your phone. But since Live Photos take up space and are otherwise pretty useless, you probably turned them off a long time ago.)
On Android, you can use Video Live Wallpaper, ZOOP GIF Lockscreen, Video LockScreen Setting or GIF LockScreen Setting.
Since this is a small market, all of these apps are a bit strange and weird. But they do let you do some neat tricks: