Any piece of intellectual property (ie songs, videos, movies, poems, painting, etc) are considered covered by copyright the moment that they are created whether you show anyone or not. The problem if you dont show them though is that, if you cant prove when something was created, and someone claims to have created it and have witnesses dating the first awareness of it before anyone can verify yours, you would lose the claim even though you created it.
Putting disclaimers and ads showing where people can buy the material that you are using does NOT allow you to use whatever you want.
You have the same copyright protection rights as anyone else whether you are a partner or not. No one has the right to use your work without your permission.
Now, in the cases where you are seeing people using other peoples work, there are a few ways this can happen. They have permission, the person who owns it is not aware of it being used by someone else, the person knows and does not mind or it has been altered in a way to allow for fair use.
Fair use does not mean that if you alter the pitch or anything like that. It means that you are using someone elses work in the context of teaching, critique, review, scholarship and a couple other reasons. In claiming fair use, you have to also prove that the medium (ie. youtube, dvd, lecture, etc) are intrinsic to your use.
One last point. Youtube is HUGE. Not every infringement is caught and dealt with but you can lose your channel if you go against youtubes terms of service which includes copyright infringement. Because some slip through the cracks, it leaves people to believe that its okay, especially when someone puts a disclaimer and claims thats why they get away with it - when the reality is, the ones that put a disclaimer that get caught are removed, so you dont witness the disclaimer not working.
I hope all of this helps. (-: