Hello.
In default Magento there is no separate option to export category structure. A workaround might be creating test product, assigning it to all categories, exporting that singe item to file. Then delete from file all the other rows except Categories.
This is not a full solution, as it only allows to get column with categories. Category settings will not available there.
In case you need to export category tree with category info and settings, you can achieve this using code or extension. If you do not have special programming skills to create or update the script and you need solution to perform 'once in a while' you can try Store Manager for Magento software that has Categories Export/Import -
https://www.mag-manager.com/useful-articles/magento-category-management/magento-export-categories/
You can check free fully-functional version of the application using the link in my signature and perform category tree export during 2 weeks without any fees (no cc or paypal needed).
