Let's say you have a very large Zotero library, from which you export a likewise large BibLaTeX file, say full-library.bib.

In your document you load that large BIB-file (because that's easy), \addbibresource{full-library.bib}, but naturally cite only a subset of works.

Now you want to share the source code for this document with someone else, and provide them with a BIB-file so they can recompile it if they wish. Can we produce a BIB-file containing only the entries that were actually cited in the document?

Yes, we can. biber has built-in support for this scenario. Just make sure your latest compilation produced a proper .bcf file, then:

biber --output_format=bibtex --output_resolve document.bcf

which will create document_biber.bib, a properly formatted BibLaTeX BIB-file containing only the subset of works from your library that were actually cited in this document.

I'm afraid I don't remember where I learned this.