How does one, "Underworld?"

[BURROW]

Im looking at the decompiled version of plr_helltower and I noticed that in the underworld, they have these oddly nice looking islands (I guess?) but these are all brushes and I thought they were props. How they do make these line up so smoothly with the displacement tool?

Note: I am aware of the "Sew" tool in the displacement thingy, but I dont know how to use to properly, ive only used it twice.

 

[Brokkhouse]

Note that decompiled displacements don't look at the same way they looked on Valve's end. The decompiler doesn't know which displacements belonged together, and will make a new brush for every single one. These were likely formed out of a single brush, and then simply shaped with the displacement edit sculpt tool.