The incumbent can have a challenger from his own party and that s the usual case.
Some states elect delegates the same way the do any other office. Most appoint them through the basic party machinery. Your local party offices may or might not have an assigned delegate to go to the convetion and they all get up and say "The great state of ______, home of ______, cast all there delect votes for ___________."
It's pretty much a formality now. However before various election reforms in the 1950's the delegates could vote pretty much the way they saw fit. Now they have to vote the way the majority of voters in there state votes.