Could you please explain the usage of 'which' both in AE and BE because I've seen that there's a difference between BE and AE.
I assume you are talking about "which" and "that" in relative clauses.
In AE, we almost always use "that" in restrictive (defining) relative clauses and "which" in non-restrictive (non-defining) relative clauses. In BE, "that" is used only for restrictive relative clauses, but "which" is used in both.
In both forms, only "which" is used when a relative clause is introduced by a preposition, whether or not the clause is restrictive.