Khilafah

Assalamu Alikum

I was going through my notes and i just wanted some more explanation since i can't find it anywhere on the web.

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Basically i'm confused on how a Khalifah comes to be? Is he appointed by the people (Like Ali), or by the preceding Khalifah (Abu Bakr did this to Umar)? Why is this different, shouldn't the Muslims pick the nominee instead of the preceding Khalifah?

Also the specific Bay'ah is given then the general Bay'ah is given. What is the point of the General Bay'ah, and what is it exactly? I read that the "scholars are agreed that in order for it to be valid it is not essential for all the people or all the decision makers to give their bay’ah." http://63.175.194.25/index.php?ln=en...QR=23320&dgn=4 So again, what is the point of the general Bay'ah?

How is specific Bay'ah done? Is there a Shura which picks the people that should be the leaders, and then the leaders decide who to give Bay'ah to?