Why do adopted daughters or sons look like adoptive parents?

That's what people say, for example, this family has no children, and there are children who are not related by blood. When a child was a child, he was a thousand miles away from his adoptive parents. However, as he grows up, children become more and more like adoptive parents. Maybe he spent too much time with his adoptive parents. I don't know what this phenomenon is and I don't understand it! Please accept it if it helps. Thank you.