Depends on the armor type. Are we talking TK armor, or Mandalorian armor, or something else?
The TK undersuit, as far as I know, doesn't have the armor directly attached to it. It's all straps and harnesses, I believe. And I'm pretty sure they wear a form fitting body glove, like under armor.
Mandos are completely different, in that their armor attaches to a flak vest, or in the case of some pieces, directly to the flight suit. Here, flight suits can be anything from actual military flight suits, to coveralls, 2 piece sets of scrubs or other work wear, that appears to be a one piece suit when all other items are worn.
