Hey there @UranHoxha, welcome to the community!
That's a good question 🙂 If that playlist isn't saved for offline use and you haven't enabled Offline Mode, then it's most likely you're using data from your mobile network. Since you are on Premium, I'd suggest downloading that playlist and any songs you like and then turning Offline Mode when you're out of range of your wifi so that you won't spend your cellular data 🙂 You can find out how here.
Let me know if that helped!
K