Migrating content with referenced taxonomy field is to be migrated by term name
i guess entity_lookup what you looking for:
here is an example from migrate_plus/src/Plugin/migrate/process/EntityLookup.php

field_tags:
       plugin: entity_lookup
       source: tags
       value_key: name
       bundle_key: vid
       bundle: tags
       entity_type: taxonomy_term
       ignore_case: true