Australia is similar to the UK. You join a political party by filling in a form and paying annual dues. You get to vote for who will represent your electorate, although incumbents will generally be unopposed for re-election. When they die / retire / get voted out it often becomes a free-for-all between party factions and branch-stacking [signing up new members who do what they are told, like block voting for your guy/gal] takes place.
Also the Labor Party, the main party of the left, has trade union representation at its policy and council level, so they have an formal role in policy and platform making.
All political parties in Australia are bleeding membership really rapidly. The only young people of any political flavour who want to get involved seem really odious and deserving of a slap. And most of the oldies are no better.
In some states nobody gets to vote for their parties nominee in any given year. While it is a more common model in minor parties, there is no requirement to have a primary or caucus in either of our major parties rules. A state convention can also be used to select delegates to the national convention.
There were several states that canceled their 2012 Democratic primaries. There were several that canceled their 2020 Republican primaries. In both cases an incumbent without a strong internal challenger was running for reelection. In states where the presidential primary is distinct from other voting, the convention route is a way to save money by not paying to hold an election.
In many states when you register to vote you indicate your party preference. You can do that without actually joining the party. In California the primary for most parties is an open primary. If a ballot is mailed to you it will be the ballot of the party you listed as your preference. But you can walk in to any voting place and turn in that ballot and ask for from a different party except the Republilkin party. To get a Republikin party ballot the party requires you to have registered with the Republikin party listed as your party of preference.
The exact process will vary, but in the end it’s almost always either one of these, depending on state law and/or party rules.
Voters might have to declare a party affiliation in advance (a “closed” primary/caucus) or they might be able to simply show up and vote in any party’s primary or attend any party’s caucus (in which case it’s “open”). In a closed contest, the deadline to register with a party or change your registration might occur well before the candidates start breaking through to less-engaged voters, thereby making it impossible for a candidate to inspire new people to join the party to vote in its primary; this is a bug to some people and a feature to others.
As everyone has said - it depends by state.
Mine (Washington state) is very much an outlier - but voters almost never give their party affiliation (it is not on voter registration forms) and supposedly the government isn’t supposed to know.
For non-presidential primaries, all candidates are on the ballot.
The candidates say which party they prefer, and voters vote for whomever you like. It is explicitly stated that a candidate preferring a party does not mean the party prefers the candidate.
For the presidential primary, the ballot has the candidates for all major parties. Voters have to promise not to participate in any other party’s presidential candidate selection process for that year in order to vote. But anyone can vote for either party if they care enough to do so.
On the one hand, it feels like it takes forever compared to other countries. On the other, there’s no one waiting in the wings to take over (as I understand it - should the Labour Party win the next election, Starmer would become Prime Minister) and so it takes about eight months to sort out who the candidates should be - especially when anyone can decide they want to be a candidate and very many people could potentially cast a vote.
But there is this leadership structure present in Congress. Now, again, there’s more time involved in switching things around, but when a house of Congress flips parties, you generally know who is going to be the one in charge of things, and most people will keep their committee assignments - just a few will change so the majority have a majority on each committee.
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