1: Ancient indians, not sure if they have a name for that people really, they were also constructed under a very long time span.
2: Mostly from nearby quarries, they´re primarily made out of limestone. there are several interresting documentaries about how they built them, how they transported the huge stones etc.
3: I´m not sure if anyone knows and my guess is it varies alot depending on size obviously, and also lot of other factors like slaves and the pharaoh and so on.
4: If they were indeed wholly or partly man made (still debating it as far as I know) it´s no real mystery, sea levels have risen and fallen over the years, istosasy in some form seems likely. There are other "sunken cities" around the world, and more will probably be found too.
5: They weren´t really. The ones you mention in Brazil for example are very different from the egyptian ones.
6: Almost exclusivley for various religious reasons, and sometimes scientific aswell. In the case of Egypt BmoreBird22 has written the explanation, graves.
Otherwise I suggest google or youtube, there are plenty on these subjects for curious people.