The best file converter I've ever come across is Format Factory. It's freeware, but be careful-- I've heard that the latest version is adware. I'm still using a version from several years ago.
I don't know what format iTunes uses, but I assume it's MOV. Format Factory can handle that normally, but there may be protection involved. I suspect that most videos like the kind you mention on YouTube are ripped or captured from DVDs, so somebody might be able to recommend a good screen cap program for you.