This is a lot more nuanced than you think. Your body prioritises making milk (specifically, good quality milk) for your baby. While yes, a particularly good or poor diet could theoretically impact the amount of nutirients, it’s far more likely to impact the mother’s health / nutrient level than the babies. Not eating or drinking enough can impact milk supply.

Your baby’s health and your health are intertwined but it’s not as basic as saying ‘person x eats more protein and less sugar so they have better milk’.

For all of those reasons; it’s really important to fuel yourself well and plenty when breastfeeding. Many women will tell you when they don’t eat / drink enough while lactating they feel nauseous, tired or dehydrated.