In the library means "in the library", "in" means "in" and "library" means "library" or "that library".

If the library has a name, add a name after "the", for example, "in the Wuxi Library" is in Wuxi Library.

If the place you want to express has a name, and the other person knows which one it is in a wide range, you don't need "that".

I hope my answer can help you, and I hope it will be adopted. Thank you!