No, natural hair extensions do not come from dead people. Depending on the quality and price, the most of them are from Malaysa. The women there brush their hair over a box and sell it to a company that is located there. They also cut there hair on occasion to sell to them. You can usually tell the Malaysa hair because of its thickness and stiffness. More high quality extensions and made from hair that is sold to the company from individuals here in USA. There are also rumors that the Malaysa hair has lice. This is also not true. Lice cannot live without a blood supply for very long.
As far as synthetic hair, it is usually stiffer and shinier. The downfall is you cannot curl it. So, you are stuck with whatever curl formation it begins with. If you try to curl it with any form of heat, it will melt and smell terrible.
To answer your "glueing" question, I'm not that informed on that.