He was the founder of our country - George Washington. After him, political parties formed based on farm or business interests. The US is unusual in that it has had but two major parties throughout our history. I mean the Democratic and Republican parties. This is what is taking place now.
Many candidates go from state to state trying to win contests to represent their party.
Many candidates begin, but the number is reduced very rapidly. We have currently reduced the number to only two candidates from each party. At the end of the primary campaigns, there are big party conventions in the fall. The winning candidates are officially announced. The only suspense these days in conventions is finding who will be selected for vice president.