It's not usually quite that simple.
If the hen is sitting on the eggs, and the chicks hatch, they can live with her outside.
But if the chicks hatch in an incubator, and you try to put them with a hen that has not been broody, the hen will either ignore them (so they die of neglect) or kill them (she might think they are food, or she might think they are intruders and try to drive them away).
Chicks that are living with a mother outdoors will eat whatever the hen finds for them to eat. In some places, that is enough for them to grow and be healthy. But more often, there is not enough stuff available for them to eat. If there is not enough food, they may starve and die, or they may stay alive but grow slowly and not be healthy.
Providing chick food is usually a very good idea.
In the first post, you said you can get "pullet starter" feed. That is probably a good choice for feeding chicks. A food labeled "starter" is usually meant for chicks to eat from the very first (when they "start" eating). If the label has feeding instructions, you can see whether it mentions what age it is meant for.