How are the 3rd and 4th steps of Anāpānasati “trained” in practice?

I understand from the Pāli terms in Anāpānasati: - Pajānāti means “to know” (as in knowing long or short breaths). - Śikhāti means “to train” or “to cultivate.” For the first two steps of the practice, it’s clear how one “knows” the breath - simply observing the natural in/out breath without controlling it, but with the effort to know it. For the 3rd step, my understanding is that one trains to be aware of the whole body in connection with the breath. However, for the 4th step, I am unclear: How does one trains to tranquil or calm the body? Does this tranquillity arise through deliberate effort, or does it occur naturally with the right effort to watch or know the breath from earlier stages? What exactly is meant by kāyasaṅkhāraṁ (bodily formation) in this context? I would appreciate clarification on how the 3rd and 4th steps are actually trained in Anāpānasati practice.