Complete "Emil's Memories" in Route A/B (started by visiting the Lunar Tear in the commercial district after you have visited Emil's shop), then complete "Gathering Keepsakes" in Route C (given at the resistance camp) as 9S after 2B's death. That scene is the final segment of that quest.

On my first play through of the game I had a moment where someone put a sword in the ground in the cave full of lunar tears in the commercial district.  It was a sort of burial for 2B.

However, on a new save file I can't figure out how to recreate it.  Does anyone know what needs to be done to trigger this scene again?  Thanks!

[Spoilers all] question