So the store itself is still located in '/', it's in '/gnu/store'. However, the location can be changed when building guix (you would need to compile it yourself) with the ./configure options --with-store-dir={PATH}. (The /var/guix path you referred to is the state path, where guix stores accounting databases and stuff for it to actually track the store)
EDIT: I forgot to mention, do not do this. If you want the store in a different location, set up a bind mount. Guix makes assumptions that would make it so you no longer could use substitutes (binary downloads of packages), so keep that in mind.[1]
[1]: https://lists.gnu.org/archive/html/help-guix/2022-02/msg0016...
However, a thread from a few years ago seems to imply that I have the wrong mental model of GuixSD and it's not "homebrew but in Guile" rather it's "GNU software all the way down" and since glibc doesn't build on macOS (according to that thread), Nix allegedly sidesteps this by making some of its packages impure and linking to libSystem https://guix-devel.gnu.narkive.com/BnGNBXUh/guix-on-macos#po...
I was happy to see that folks in that thread got GuixSD working for docker images, so that can still be a win, but my current heartburn is not building docker images so I am not the target audience for that