In the 6 preliminary steps, it involves sitting down cross legged,
and straightening the body 🏃 (kāya).
It's the same entire body (kāya) that you train yourself to become sensitive to in step (3), the sensation of breath energy that permeates your entire physical body 🏃 and awareness.
kāya doesn't just suddenly transform from a physical body, into a "body" of breath devoid of a physical body. That's the lie that late Theravada tries to feed you, as well as Ajahn Brahm. Don't believe the lies, read what the suttas actually say about what it means for the breath to be an aspect of the body: