I'm reading about Monte Carlo tree search. I'm not understanding when one moves from the selection to the expansion phase. Suppose I reach a node in the decision tree that has 50 children (positions I've explored at least once), but there are 150 possible moves. So there are 100 moves I haven't examined at all, so they have no branch. Am I in selection or expansion mode?
As a second question, is the typical thing in the rollout phase to make moves purely randomly, since these are situations I presumably have no way of evaluating, because I've never seen them before?