It's all about masking. Chroma keying is merely a convenient way of keying out the background. Alternatively, and like the Star Wars kid video, someone would have had to rotoscope him out. This means they would literally have to draw a mask around him on every single frame. It can takes weeks.
If you are interested in this kind of thing I can recommend this book as an introduction
Amazon.co.uk: Compositing Visual Effects: Essentials for the Aspiring Artist: Steve Wright: Books
It goes through all the basic concepts you will need. It's not in any way a "how to do this in After Effects" type book or anything like that. Itr'#s a concepts book and that is what I think you are asking. i.e. how and why does thiss stuff work.
What s/w do you have available. you may already have some half decent keying s/w available to you. You neve know.