

You don’t lose your copyright just for posting it in a public space, even Reddit. But you do give reddit a perpetual, non-revokable, transferrable license to do basically whatever they want with your IP :
Found here under “your content”: https://redditinc.com/policies/user-agreement
With Lemmy all that language of perpetual, non-revokable, transferrable goes away to my knowledge. You still wholely own your own IP if you decide you don’t want it on Lemmy anymore.
Written works are tangible and so have a copyright upon creation, just like the video in your example. That recording posted online “publicly” where anyone can see it free of charge wouldn’t change its copyright. Also private internet sites really aren’t “public” space in the ways most laws would define it, because it’s a server hosted by a private individual. We’re in ruud’s house right now so-to-speak. He has every right to censor us and show us the door if he so chooses.
By posting or commenting here (or on reddit for that matter) we don’t fully waive copyright to IP. If I write a unique poem here and some random person plagiarizes it and sells it I could still sue. But on reddit, if reddit decides to publish a book of “Best of reddit poems” or transfer that license to someone else I’m shit out of luck. On lemmy without the legalese I stand a good chance in court revoking the assumed license of my work and having a positive legal outcome.