Couldn't have put it better myself.
Pretty much every community is watched very closely by at least a few super moderators and admins, which should be fairly clear considering how many there are. Members who contribute a lot and are generally suitable for a moderation role will be noticed and of course be first in line when some new moderators are required.
Worst thing you could really do is join or participate in a community purely to become a moderator. Doing that is liable to make you come across as artificial and bland, which isn't really what anyone wants in a moderator. Having a good, fun personality is just as important as the ability to make decisions.