You’ll get rewards once you finish the available amount of matches (maximum of 9) in the competition. You can also choose to end your current Play-Offs event at any point, even before completing all your matches, allowing you to collect the available rewards immediately. If you don’t finish all your Play-Off matches before a UT Season ends, you’ll automatically get whatever rewards you have earned from the matches that you did complete.
You can check your progression in-game and see the requirements and rewards for each Rank.