The difference between people being born into a culture and people learning a culture is really another variation of the old nature versus nurture question. Let's look at each of these perspectives to help you better understand the situation.
Those who argue that people are born into a culture mean that from the time people are infants, they are surrounded by their culture. It becomes a part of their being. They don't think about learning culture. They absorb it. It is natural to them, normal, a part of who they are and how they live.
Those who argue that people must learn culture refer to the many elements of culture that are taught. Children learn the stories and songs of their culture, for instance. They learn how to prepare certain foods. They learn what makes their culture unique and valuable as they experience traditions and listen to their elders teach them their heritage.
Actually, we could make a case that people are both born into a culture and learn a culture. They assimilate their culture unconsciously, and they learn it consciously. Both elements are in operation at the same time.