Ryan is not from the US and I’m not sure how his party system works
WeTo put it as simply as I can… In a General Election. We just vote for our local MP. The Party with ththe most MPs wins and must form a government. We don’t directly vote for the Prime-Minister.
In the US, a primary is an election held by a party to determine who will represent that party in the upcoming general election against the other party or parties. Hillary beat Bernie in the Dem primary and Trump beat a billion other Republicans in the Repub primary.