How can I export and then import taxonomy terms?
The line between config/content can be blurry since there are many use scenarios. Drupal assumes a default scenario where terms are user-edited content that's not part of the CMS configuration.
Taxonomy terms are content entities, unlike taxonomy vocabularies, which are config entities. Hence, the D8 configuration export/import will not work for taxonomy terms.
There are various modules that will let you import content (and thus taxonomy terms). Some are more general-purpose (e.g. the migration framework, the content sync module), others are specific to taxonomy (e.g. taxonomy import module). Research solutions and figure out what works best for your scenario.