'which' in this sense is used in relative clauses with inanimate antecedents ( a word or phrase to which a pronoun refers ).
the preposition 'in' before it, indicates that the pronoun refers to something inside, within ..
"The house , in which he lives, is very big ."
Here the pronoun 'which' refers to the preceding inanimate noun ' house' and adding 'in' before indicates that the action (he lives) is happening within ,inside it.
You wouldn't need to use 'in' if the sentence was like:
"The house ,which he owns, is very big."
"It is a healthy environment, in which the youth can develop individual and group awareness."
Here 'which' refers to the 'environment' and 'in' indicates the event is happening within this environment.