there has only been two, khomeini and khamenei. khomeini is persian but some say he is indian because his grandparents lived there and in his poetry he writes his pen name as something indian lets say. khamenei is azeri. no resentment for those reasons that i know of.
what he controls is not clear, i think we can say everything needs his permission, but sometimes he will give permission if he thinks the people support it, like the negotiations with the US etc for the iran deal. he was opposed but people voted for rouhani so he gave his consent, but it was clear he didnt like it. now the irgc and other factions are powerful so it's not clear that he can just give orders and everyone will listen. rouhani has limited powers, cannot go against the leader but can set an agenda and try to implement it in majlis (parliament), but hardliners control all the institutions from judiciary to security to council of experts and guardian council (control who can run for elections, what bills can be passed, etc).
as for who chooses, there is a body that can supposedly fire a leader, but it has never done so and half of it is appointed by the leader himself so it's unlikely it ever would.
i think not enough is spoken about how khomeini's idea of the guardian of the jurist has very little support about shia scholars, even in iran none of the top shia scholars supported it. he started by saying he would just advise on things but then, of course, slowly became a dictator instead.
