Question: how to set a purchased song as ringtone on iphone 11?
Answer: Open the iTunes Store on your iPhone.
Go to "Genres" or "More" > "Tones" > "All Tones".
Find or search for the ringtone you like. ... 
If that's what you like, tap the price of the tone and choose "Set as Default Ringtone".
Pay for the ringtone.