Quarto book chapters and sections are automatically numbered (for cross-referencing), however you can also specify that some parts of the book should remain unnumbered.
Meaning that, you can reference any section by default in books as they are numbered by default, but you can create unnumbered section that won't be referencable using @id syntax.
If the section you want to link too is in the same document, you could probably use section linking using [text](#id-of-section). This should work from within the same doc.
Cross referencing works across all documents but it requires numbering.
Thanks! Helpful, and it's obvious now why I can't refer to "Chapter #" when the chapter is unnumbered. I think I'll solve this problem by putting it an appendix in the quarto yml and referencing as [Appendix -@sec-setup].