In the modern us political system, what is the main system political parties use to nominate candidates? primary caucus general election party appointment

In the modern US political system the main system that political parties use to nominate candidates are primary elections, however, caucus' are also used in certain states. In a primary the vote is cast by a secret ballot in order to decide the candidate of the party. In a caucus members of the party meet in an open meeting and discuss who they will select to be the party candidate for that state.