How they built them

[savagecreature]

Welp, there are many answers because there are many droids (obviously).  I only know some. There are folks much more knowledgeable than myself, but I'll give you a small bit of what I *do* know:

The interrogation droid is actually (at least half of it) an R2 unit. The upper part of it's body is a repurposed R2 dome, as can be seen in the image below (click for larger version)

As indicated by the image, some of other parts have been identified as medical implements, others are assumed to be other found objects from various things that they glued on there.  R2 and the other astromeches were (almost) entirely custom fabricated for the films, as was C-3P0. The background droids were more often assembled from found objects or parts taken from droids they already had. Both the Death Star droid RA-7 and the bounty hunter droid 4-LOM incorporated many parts originally made for C-3P0.