“How does the young-earth theory explain that we can see stars millions of light-years away? How would the light have reached us?”

Since God made the sun, moon, and stars “to give light upon the earth” (Genesis 1:14–18), those lights would be immediately visible on earth. They fulfilled their purpose on the day God spoke them into being, because He “saw that it was good.” No doubt God also made Adam as a fully-grown man—perhaps with the appearance of being 30 years old, even though he was only minutes old. Likewise, herbs and trees were already mature and fruit-bearing, to provide a ready supply of food. That would be the case with all of His creation.