Underpass Fortnite is the location in finding Fortbyte #22. The inclusion of Fortbyte pieces were added in Season 9 giving players something else to find throughout the whole season.
Like most of them, they are revealed over time.
Underpass Fortnite: How to Find the Fortbyte #22
It's in Neo Tilted, towards the center. It's basically the only underpass in the whole game. You should see the locked Fortbyte floating in the air toward the western side of the underside. To collect it, just use your Rox spray anywhere in the general area and it will unlock. However, you can only unlock the Rox spray once you hit level 5 of the battle pass.
Photo courtesy of Epic Games