Will registration protect all my rights when I upload my content to Youtube?

This is a recurrent question, nevertheless not frequent, but worth mentioning. Short answer is: No. A longer approach requests a bit more text:

1) As author of an original work you have all the rights upon that work whether you register it on official or private institutions or not. Registrations are declarative: Thet do not give any rights as you already have them all (still there are some remarks you should consider).

2) This being said, when you agree to use a publishing platform such as Youtube, you are also accepting their terms of use; which means you are agreeing to giving them some rights upon your works. If you don’t agree with those conditions, you can either try to negotiate them or not use the service at all.

3) That you have now all the rights of a work doesn’t mean you will always keep them because, yes as you guessed, if you voluntarily give some of them away, you no longer have them all. No matter you registered or not. Both are separated issues.

This said it’s always highly convinient to register your works because even if you plan to give some rights, there’s always the possibility that either Youtube (unlikely) or some other third parties might not respect the terms of use of that agreement. In those cases to have an early authorship evidence is always key in order to defend your rights and or to negotiate.