What is the difference between Zwischen and Unter in German, and are they interchangeable?
unter/zwischen allen diesen Jungen wählte ich dies aus.
"Unter" means "among" while "Zwischen" means "inter" (the inter prefix..) or between - you could have looked it up i think.. Most times, they are not to be changed with each other.
changing "unter" in the first example is an odd way to say you are "between" them, like standing close between two friends.
Adding your example:
More common use would be "Aus all diesen Jungen"..