Steam is by far the biggest player in PC, but there are many outside it like those you mention, or Riot. Plus some Asian focused ones.Also Epic Games is probably Valve's biggest competitor: They have Fortnite, Fall Guys, Rocket League, all Mihoyo games as "exclusives" to their storefront, and they also compete with Steam for 3rd party spend ala Xbox and PS.
I remember that during a decade or so the top 1 of the top grossing PC game ranking was frequently changing between LoL and Dungeon Fighter Online, when none of them were on Steam.