What is Khilafat-e-Ahmadiyya and why is it needed?

Why did the Khilafat of Holy Prophet^(sa) did not continue and there was need for Promised Messiah^(as) to restart it?

Promised Messiah^(as) was appointed by God but the Khalifa is elected by human beings. Can you shed some light on this?

Why do some people can choose a Khalifa?

Are people trained to be Khalifa?

Is Khalifa like a Pope?

What role does Khilafat play in the life of an Ahmadi?