Much of the WWDC content (understandably so) covers how to use localized strings in code but a piece of the puzzle that's missing for me is the exported and imported .xcloc files.
How do translators use and modify an exported xcloc file to provide translations? Is there a popular 3rd party software for this step?
The Localization Catalog (.xcloc) is a package format that contains an XLIFF (XML Localization Interchange File Format) file with all of the strings in your project. XLIFF is an industry standard format that your localization provider should be familiar with. There are also many third-party tools and services that read .xcloc and XLIFF files.