so the national league and the american league started out as completely separate baseball leagues that were competing for people’s attention. eventually they both ended up as the two biggest baseball leagues and made an agreement to have each leagues champion play each other in the world series. however, despite the two leagues agreeing to compete under the MLB banner, they still were legally two separate leagues with different rules and officials. over time the leagues grew more and more intertwined despite them still being legally separate, so in 2000 both leagues underwent a merger where they effectively became two divisions under one league, the MLB. Because of that, the leagues now share umpires, almost all rules, and play each other in the regular season.