https://understanddrupal.com/articles/migrating-taxonomy-terms-and-multivalue-fields-drupal

multivalue_field_syntax.yml

process:
  field_photos/0/target_id: source_fid_first
  field_photos/0/alt: source_alt_first
  field_photos/1/target_id: source_fid_second
  field_photos/1/alt: source_alt_second
  field_photos/2/target_id: source_fid_third
  field_photos/2/alt: source_alt_third

node_migration_destination_and_dependencies.yml

destination:
  plugin: 'entity:node'
  default_bundle: article
migration_dependencies:
  required:
    - udm_dependencies_multivalue_term
  optional: []

node_migration_process.yml

process:
  title: thoughtful_title
  field_tags:
    - plugin: skip_on_empty
      source: fruit_list
      method: process
      message: 'No fruit_list listed.'
    - plugin: explode
      delimiter: ','
    - plugin: migration_lookup
      migration: udm_dependencies_multivalue_term

node_migration_source.yml

source:
  plugin: embedded_data
  data_rows:
    - unique_id: 1
      thoughtful_title: 'Amazing recipe'
      fruit_list: 'Green apple, Banana, Pear'
    - unique_id: 2
      thoughtful_title: 'Fruit-less recipe'
  ids:
    unique_id:
      type: integer

taxonomy_term_migration_destination.yml

destination:
  plugin: 'entity:taxonomy_term'
  default_bundle: tags

taxonomy_term_migration_process.yml

process:
  name: fruit_name
  description: fruit_description
  parent:
    plugin: migration_lookup
    migration: udm_dependencies_multivalue_term
    source: fruit_parent

taxonomy_term_migration_source.yml

source:
  plugin: embedded_data
  data_rows:
    - fruit_name: 'Grape'
      fruit_description: 'Eat fresh or prepare some jelly.'
    - fruit_name: 'Red grape'
      fruit_description: 'Sweet grape'
      fruit_parent: 'Grape'
    - fruit_name: 'Pear'
      fruit_description: 'Eat fresh or prepare a jam.'
  ids:
    fruit_name:
      type: string