Drupal 8 migrations by example

mbe_book_paragraph.csv

mbe_photos.csv

mbe_professors.csv

migrate_plus.migration.mbe_book_paragraph.yml

id: mbe_book_paragraph
label: 'Import book paragraph'
source:
  plugin: csv
  path: ../migrate/mbe_book_paragraph.csv
  header_row_count: 1
  keys:
    - 'Professor ID'
process:
  field_mbe_title: Title
  field_mbe_author: Author
destination:
  plugin: 'entity_reference_revisions:paragraph'
  default_bundle: mbe_book
migration_dependencies: null

migrate_plus.migration.mbe_photo_field.yml

id: mbe_photo_field
label: 'Import photos of professors'
source:
  constants:
    SOURCE_BASE_PATH: 'http://agaric.com'
    DRUPAL_FILE_DIRECTORY: 'public://professor_photo'
  plugin: csv
  path: ../migrate/mbe_photos.csv
  header_row_count: 1
  keys:
    - 'Professor ID'
process:
  destination_filename:
    -
      plugin: callback
      callable: basename
      source: 'Photo URL'
    -
      plugin: skip_on_empty
      method: row
  destination_full_path:
    -
      plugin: concat
      delimiter: /
      source:
        - constants/DRUPAL_FILE_DIRECTORY
        - '@destination_filename'
    -
      plugin: urlencode
  source_full_path:
    -
      plugin: concat
      delimiter: /
      source:
        - constants/SOURCE_BASE_PATH
        - 'Photo URL'
    -
      plugin: urlencode
  uri:
    plugin: file_copy
    source:
      - '@source_full_path'
      - '@destination_full_path'
destination:
  plugin: 'entity:file'
migration_dependencies: null

migrate_plus.migration.mbe_professors.yml

label: 'Import professors'
source:
  plugin: csv
  path: ../migrate/mbe_professors.csv
  header_row_count: 1
  keys:
    - 'Professor ID'
process:
  type:
    plugin: default_value
    default_value: mbe_professor
  title:
    plugin: concat
    source:
      - 'First Name'
      - 'Last Name'
    delimiter: ' '
  field_mbe_biography/value: Biography
  field_mbe_biography/format:
    plugin: default_value
    default_value: basic_html
  field_mbe_department:
    plugin: entity_generate
    source: Department
    entity_type: taxonomy_term
    bundle: mbe_department_vocabulary
    value_key: name
    bundle_key: vid
  field_mbe_photo/target_id:
    plugin: migration_lookup
    migration: mbe_photo_field
    source: 'Professor ID'
  field_mbe_photo/alt:
    plugin: default_value
    default_value: 'Professor headshot'
  mbe_book_paragraph:
      plugin: migration
      migration: mbe_book_paragraph
      source: 'Professor ID'
  field_mbe_favorite_books:
    plugin: iterator
    source:
      - '@mbe_book_paragraph'
    process:
      target_id: '0'
      target_revision_id: '1'
  field_mbe_experience_date:
    plugin: format_date
    source: Date
    from_format: 'F d, Y'
    to_format: Y-m-d
    timezone: America/Managua
  field_mbe_coworking_address/address_line1:
    plugin: default_value
    default_value: 'It is a secret ;)'
  field_mbe_coworking_address/locality: City
  field_mbe_coworking_address/administrative_area: State
  field_mbe_coworking_address/postal_code: ZIP
  field_mbe_coworking_address/country_code: 'Country Code'
destination:
  plugin: 'entity:node'
migration_dependencies:
  required:
    - mbe_photo_field
    - mbe_book_paragraph
  optional: {  }

@summitims the full example modules are available at https://github.com/dinarcon/ud_migrations and blog posts with the explanations at https://understanddrupal.com/courses/31-days-of-migrations/

I have not added D10 compatibility yet. I expect minimal changes are necessary, but I have not have a chance to do it yet. Keep an eye on the repo and on the site for updates.