which specific instance is being fererred to

Hi everyone!

I am wondering if the word "which" is a relative pronoun in the way it it used in the following sentence:

1.-They do not know which specific instance is being fererred to.
The usage of "which" in this sentence is not familiar to me.

Is "which" a determiner?

Consider these sentences (relative usage of "what") :

2.-I do not know what it is.
3.-I do not know what you mean.
4.-I do not know what you want.
5.-Bring me what you´ve written!

Can I use "what" instead of "which" in the first sentence?

1.-They do not know what specific instance is being fererred to.

Would it be correct to say "I do not know WHICH you want"?

Could you exaplain what the diference is between the usage of WHICH and WHAT in my examples?

Sorry for the awkwardness

Thanks in advance