Question: destiny 2 how to go over level 20?
Answer: Focus on only the story missions and ignore all other activities.
Don't bother with the enemies in missions. ... 
Playing in a 3-player team is faster than going at it solo.
Between main missions you'll have to level up (reach level 6 / 11 / 15 to access new planets and missions). ... 
The Crucible also gives XP!