The best food for a baby is mother’s milk. However, in the absence of that, the baby can be given formula (nowadays you get vegan formula too). Once it starts teething, you can add solid food starting with raw mashed fruits. Later you can add cooked foods like rice and potatoes. Introduce new foods one week at a time to give the baby time to adjust.
However, the concern should be why the mother isn’t able to produce milk. We need to ensure that the mother is healthy even before and through the pregnancy and is consuming the right nutrition.