@AskingQuestionsAllTheTime
Are there actual rules about how you can/may vote? I mean, if you are registered as a voter with the Republican Party, can you vote Democrat all down the slate? Because if so, there might be a bit of a shock for the GOP on the day, if a lot of people they have assumed are safely republican vote for Biden. Presumably this hundred McCain staff members were registered as Republican until this point, for instance, but there may be a lot more who feel the same but haven't said so.
General elections are unrestricted, you may vote for a candidate regardless of party. Your ballot lists every candidate selected in the primaries plus usually room for a 'write in'.
Primaries are different. Primaries choose candidates to run against the opposition in the general election. Most states have 'closed' primaries which means you are given a ballot listing only the candidates of your particular party. This is to stop non-party members voting in an unsuitable candidate for the 'other' party.
I expect that the McCain staffers are still registered Republicans. There are still some good'uns out there, even if I disagree with many of the things they believe in. But I do believe the GOP may be surprised when the final numbers come out to see how many voted Biden.
Ballots are secret so no one will ever know specifically who voted for whom. But they do know the total number of registered voters by party, so I expect it'll be pretty apparent.