1. How does necromancy exactly work?

I remember reading some books about how necromancers put souls into some other bodies to "revive" them. So what happens when necromancers reanimate a corpse? Is his or her own soul dragged back to the body and bound into it temporarily, or just some random souls get trapped into a random corpse? Or do they control the body directly without a soul inside?

2. About Hannibal Traven.

It is explained that he sacrificed to fill the colossal soul gem, for preventing King of Worms turning the CoC to a worm thrall. How does this work? This question is kind of related the first one. Why carrying a soul gem with Traven's soul would help the CoC?

So Mannimarco can turn a living person directly into an undead without killing them. I wonder what is going on in the process and how does the colossal soul gem affect it. And what makes Traven's soul special? Why did Mannimarco want HIS soul and why HE is the one need to die to help CoC?

3. About Vaermina and necromancy.

There is little information about this daedric prince. From Oblivion and Skyrim I feel like she is connected with necromancy somehow. In Oblivion, you have to offer a black soul gem, which is typically a necromantic artifact, to active the shrine. In Skyrim, there are some clues about it in Erandur's dialogue. He mentioned when he was a priest of Vaermina he learned black magic, worked in a catacomb of a fort, and he seems have a good amount of knowledge about necromancers, which indicates that the priests of Vaermina need to do something with necromancy.

But I wonder why. The realm of Vaermina is dream and nightmire. What's the purpose to perform necromancy to serve her? And how necromancy is connected to her realm？

I appreciate any ideas and discussion on the questions.