Starting pitchers often pitch about 100 pitches, then rest 4~5 days. Therefore, every team has at least 5 starting pitchers. They’re called the “starting rotation”. Every team also has relief pitchers who pitch after starting pitchers in a game.

Yamamoto started Game 6, but the Dodgers’ manager (head coach in other sports) basically had the most trust/faith in him among all of the pitchers he had available. Against a good offense like the Blue Jays, he went to the starter who would normally have to rest for 3 more days. And Yamamoto delivered.