Do most teens identify with more than one cultural group, and how does an observer learn to understand another culture?

Absolutely, teens would identify with more than one cultural group, as all humans do. Let's assume we are discussing an American teenager, who was born in Brooklyn, New York, to a Jewish mother and Colombian immigrant father. This teenager identifies as an American, a New Yorker, a Brooklynite. Depending on what religion the teenager belongs to, if any, they may identify as part of that religion's culture. They may identify with a certain group as a result of their sexual orientation. They may have interests that make them part of a subculture, additionally, such as cosplaying.
It becomes important to differentiate between culture and ethnicity in this teenager's case. They may or may not identify as culturally Jewish, if they do not partake in Jewish traditions. However, they would identify as ethnically Jewish based on their mother's heritage. They may or may not identify Latinx, or Colombian, culturally, for the same reason; but would be Latinx ethnically.
An observer learns to understand another culture through what sociologists refer to as sociological imagination. Sociologist C. Wright Mills formed this theory for observing other cultures in the 1950s. Mills suggests that all of our observations of other cultures are tainted by our own cultural programming. The only way to truly get a sense of another's culture is to imagine oneself as part of that culture.
By anchoring our imagination in a different culture, ideally we can suspend our own cultural prejudices and appreciate why and how other cultures are as valid as our own. A great example of engaging in sociological imagination is to look at living conditions. Let's contrast a Mongolian Steppe home, a yurt, to the Brooklyn brownstone our American teenager lives in. Through understanding the lifestyle and landscape of Mongol tribes, the teenager can imagine living in the Steppe climate necessitates close quarters and shelter suited to nomadic life.