Catalogs are not single files, they are folders containing databases, many auxiliary files and subfolders.
When you have a catalog opened, you use the menu Help >> System Information which gives you the location of the catalog folder for that current catalog. You can move or copy the whole catalog folder elsewhere. Since catalogs don't contain any photo files, only links to their real location on your drives, moving or copying the whole folder keeps all the links unchanged, but that does not move the files themselves.
So, if you mean moving both your catalog (the organization) and your image files, you are wanting to move both as explained in that help doc:
You create a backup folder including everything (catalog pieces and all image files) to be able to restore everything on the same location or a new location of your choice. It's not a catalog simple copy and can't be used for anything else than restoration.
If your question is which 'catalog' to move, if you have several ones, that's up to you to choose. By default the last used catalog is backed up. For other catalogs, you have to specify which ones you want to backup in the backup settings.