I don’t know much about UK elections, but from what I see on Wikipedia it deploys different voting systems and even fptp is really a fptp system + party list so not exactly the same.

In US there truly is just a single election where you give a single vote for the candidate and whomever gets more votes wins.

Yes, there are primary elections too, but those aren’t real elections, they are elections run by the parties to pick up their candidate. They actually could not hold election and just pick up the candidate themselves and that’s what they often do for a second term.

The way it works with FPTP in US is that it naturally forces two parties, as you generally are forced to vote against someone and not for someone. This is because of there are two good candidates and one bad, the vote splits and the bad candidate wins due to spoiler effect. So people try to predict which candidates will likely win and vote for the lesser evil.

So no one in 3rd party has any chance, and generally most of the times the people who are running 3rd party are just pathetic.

Sometimes when a serious candidate runs in 3rd party it generally spoils for the candidate with similar views. That’s how Bill Clinton won against HW Bush