Bendel under the shirt over the woolen טלית קטן, gartel over a jacket/kapota when married.
FTFY.
Though I would assume the Zirkinds who wear their ט"ק over their shirts with a vest, might not use a bendel.
The primary function of the Bendel is to keep the טלית קטן spread out while wearing it. Secondary function to act as a gartel, tertiary function for carrying keys on Shabbos.