For a quick explanation of this step, step 21, basically, this is a continuation of the previous steps and what you will need to do is quick to complete.
Destiny 2 Beyond Light Step 21 Guide
As Schavan told you in the previous step, here you will have to defeat the Fallen at the Cosmodrome.
When you get here the first thing you need to do is jump to the marker, on the first floor of the building in front of you and enter the first door on the right.
Then, go right then go left and skip the rubble and continue straight through the doors. You will then encounter an open space with a bunch of enemies.
Beat them and in the left corner of this space, you will find a door. Then enter the next door on your right and beat your opponents in this room.
Keep going straight and just follow the direction of the corridors. You will now come across an open space where you will run into enemies. Note that this robot will be quite difficult to beat.
Jump to the buildings to protect yourself from the shots of the opponents and at the same time use the cover to eliminate them.
Use that strategy and after a while, you should be able to eliminateall the robots and other enemies.
And that completes this step!
ALSO READ: Icarus: How To Make Steel