Rather shamelessly in fact.
They did copy the infinite video scroll from TikTok, among other things.
Isn’t that just a different form factor for Instagram? They already had an infinite feed of content, many of which videos; TikTok made it full screen
Tiktok's UI choices (auto-play, full screen video, default feed, swipe instead of scroll, etc.) were distinctive enough to be considered something novel.
Reels are different from Stories, but yes, both features were copied from TikTok and Snapchat.
Not originally. Korean social media apps had them before snapchat.
Vine was around for quite a while too. well, 2013-2016 at least.