Everyone who is registered as a Democrat went to polls and voted for which Democrat they wanted to represent the Democratic party in the General Election.
Out west, things get murkier. At least in my state, you don’t register as a member of a particular party at all, so technically everyone in the state is an independent. At primary time, you basically decide right on the spot which party’s primary you’d like to influence this year and choose that ballot (or attend that caucus, or both because yes we’re weird that way).
You have primaries AND caucuses??? How does that work?