Physically, chestfeeding is exactly the same as breastfeeding. Hormones trigger the body to make milk, the milk travels through glands and ducts ending at the nipple to feed a baby
Chestfeeding is used in the community of people who have recently given birth but don’t identify as women. They may be transgender people who were assigned female at birth but who now identify as men, or people who are non-binary
People who do identify as female may also prefer "chestfeeding" because of physical or emotional trauma related to their breasts. To them, the word "chest" is not as triggering as "breast" may be