When I was researching copyrights I found it was as slartabartfast was saying, technically it is copyrighted once it is created but can't really be enforced until it is registered. I really never had anything that I felt the need to register.
As to youtube and copyrights, you have to be careful as there are trolls that will claim copyright trying to scam ad dollars, royalty money scam or because they merely think you stole it from them. Unfortunately, getting a take down is a lot easier than contesting one.
There was an issue with some royalty free music in which the bots were seeing some of the same loops. Since one creation using the loops may have been copyrighted, a song using those same royalty free loops were tripping the bots subjecting them to a bogus takedown notice.
As far as 'stealing'... I had an odd one. I did a video countdown leader that I uploaded to youtube for a contest. For some odd reason it went viral in Brazil (440k views). Apparently somebody in Brazil was telling huge groups of people that he did it. I then started getting a rash of comments in portuguese. According to google translate, it was a bunch of angry people wanting to know why I stole HIS work.