Typically, for any game that has a campaign, I would consider completing that completing the game.

That doesn't mean you can't continue to have fun in endless modes or multiplayer. That's a different orientation.

For multiplayer games, there's no completion really. Play the tutorial? All maps once? Win once? Ranks? Endless leveling progress? All achievements? None of those really fit. There is no completion to a game without designed, completable progress. If there's a max level, one could consider that a kind of completion. All achievements may subjectively fit too.