Guilin is a beautiful city in which I grew up.
The sentence actually can be rewritten in two other forms:
1. Guilin is a beautiful city. I grew up in Guilin.
2. Guilin is a beautiful city. In Guilin I grew up.
The phrase “in which I grew up” in English is a Prepositional Phrase.
Structure of Prepositional Phrase is a preposition (“in”) followed by a noun, or noun phrase (“which I grew up”).
In function, Prepositional phrase is a qualifier which describes or gives more information (it is the city “which I grew up”) about the noun “Guilin”.
(Note: If you want to delve further into the function of “which”, here it is.
In English grammar, the word “which” here is a “relative pronoun”.
The function of the relative pronoun is to introduce a clause (in this case “I grew up”) which describes further (“the city in which I grew up”) the noun preceding it (in this case, the noun is “Guilin”).
I hope it answers your question.