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). It’s not quite an open primary, but we love open primaries out here and it’s the closest we can get to that ideal without the parties suing the crap out of the state. The parties claim they can restrict primary access to their own members; the public reminds the parties who’s paying for these elections in the first place, and a tense standoff is maintained.
Now some of that independence is obviously fictional. We have more self-reported independents (as opposed to the meaningless registration numbers) than most states, but lots of those are leftists who are choosing between third parties and Democrats, and rightists choosing between third parties and Republicans. The percentage actually choosing between Democrats and Republicans probably isn’t that different than other states, although we do have a lot of ticket-splitters, and we were one of the last refuges of the fabled liberal Republicans before they went extinct (or more accurately became Democrats).
We also have a long history of messing with the primaries of other parties. Since the primaries are occasionally just formalities with a single plausible candidate, and you can choose at the last minute which primary to vote in, people often talk about crossing over to the other side to vote for the weakest/craziest candidate on the other side, to improve their own party’s chances. However, that sort of talk has lately fallen out of fashion since it makes the situation a lot more dire when your party loses. Now people talk about crossing over to vote for someone they could stomach, if not vote for, which I guess is moderation coming back into fashion.
I’m registered independent in MA and it’s the similar for me, pick which ballot you want when you show up. Haven’t done the “vote on the other side” thing though.