The reason you need "in which" is that in these sentences a preposition is needed. Compare these sentences:
"This is the city I grew up in; This is the city in which I grew up." These mean the same thing, as does your example 1, and my amendment in blue above.
Here are some more sentences that need a preposition:
"This is the table from which I took the book; This is the table which I took the book from." You can see that "This is the table which I took the book" doesn't work.
"This is the man to whom I owe the money; This is the man [whom] I owe the money to"
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