CGP Gray has a good explanation how FPTP works and how it breaks down and ends up typically collapsing down into a 2 party system or is wildly unrepresentative in multi party systems.
Basically fptp is a winner take all system, whoever gets the most votes wins and beats everyone else in the race. It doesn’t matter if you won by 1 vote or by 1 million votes the result is the same, you won. So if one party can maintain just a slim plurality across numerous districts, they win those districts despite not getting the majority of votes and everyone else’s votes essentially do not count in the greater whole.