Super Smash Bros. Melee didn't ship with online features—it released in 2001 exclusively for the GameCube. The Dolphin emulator changed that, allowing players to compete online in what is still considered the best competitive Smash game.
Those online features are now greatly improved thanks to the development of Slippi, a fan-made plug-in for Dolphin.
On the Switch, Super Smash Bros. Ultimate doesn't have rollback netcode, which now makes it far less viable for online competitive play compared to its much older (emulated) GameCube sibling.Smash pro Leffen outlines the benefits of Slippi in the video embedded below, and if you're wanting to install it yourself, there's also a tutorial here.