In Jainism, dress code for sadhus varies by tradition:

• Svetambara sadhus: They typically wear simple white clothes. This usually includes a light covering and may include a loincloth or minimal undergarment as part of their basic monastic attire.
• Digambara sadhus: They practice nudity as part of their renunciation, so they do not wear any clothes, including underwear.

So, whether underwear is worn depends on whether the monk belongs to the Svetambara or Digambara tradition.