It's blurred, you're right.
Some will say it's by beating those better than you, others will say it's by levels, and still others will have other answers.
Some truth in all of this, depending of a player's actions and platform.
In this case, it's not matchplay (there was none), it's probably not credit wins (none visible), so it may be Levels, due to XPs stacked in mobile games.
I agree that it's more important to have fun!