The new and upcoming Halo Infinite game from 343 Industries will not feature an XP system for the multiplayer.
Instead, for the XP system to level up, you will have to complete different challenges as written by this 343 employee.
But there is a problem. In the Tech Preview of the new Halo Infinite, players could run out of challenges and therefore not level up.
This issue probably will be fixed in the final version of the game.