Portals can tell the system which file picker to use.
If you are using Nemo, you can set Nemo's file picker.
sudo nano /etc/xdg/xdg-desktop-portal-gtk/gtk-filechooser.ini
This will offer to create the file, since it will not exist in Zorin OS.
Content:
[FileChooser]
FileManager=nemo
Save the file - restart to test. Bear in mind, that with Gnomes tight integration and the sandboxing of xdg-desktop-portal, this may not work with all apllications.
Zorin Core users might also just use the older file picker:
gsettings set org.gtk.Settings.FileChooser gtk-file-chooser-backend "gtk2"