QuarkXPress to InDesign

Check out Markzware's OmniMarkz software.

https://markzware.com/products/omnimarkz/

(Warning--not inexpensive.)

What version? InDesign can open 3.3.- 4.1 file from either platform (works well, btw). If you have newer than that, in addition to selling utilities to convert newer Quark files to InDesign, markzware also has a conversion service where they do the work for you priced according to the size of the file. That might be a better option for you if you just have one or two files. I had them do a client's huge Publisher file for me a few years back and was very satisfied with the results.

If it's QXP 6.x or less I can probably give it a try for you as I have an old Q2ID plugin on an old Mac system here.

There is some continuity, as older versions of ID could open certain file types created by certain QXP versions.

I suggest you simply try it.

If not, there are both conversion apps (which may be too pricey for a one-shot job), and services. A few of the folks on here have multiple older systems in play just for this kind of conversion as well.