If you participate in Playfull, you are automatically enrolled in the Battle Pass and will earn XP for everything you do. If you earn enough XP, you’ll level up and unlock a reward for every level you reach.
The Premium Pass is an upgraded Battle Pass. For US$10 (or MATIC equivalent), you can upgrade to the Premium Pass and unlock every reward available for the Battle Pass season. You can buy the Premium Pass any time during the season!