You should be using where, select is a projection that returns the output of the statement, thus why you get boolean values. where is a filter that keeps the structure of the dataframe, but only keeps data where the filter works.
Along the same line though, per the documentation, you can write this in 3 different ways
// The following are equivalent:
peopleDf.filter($"age" > 15)
peopleDf.where($"age" > 15)
peopleDf($"age" > 15)