https://understanddrupal.com/articles/understanding-syntax-drupal-migrations

process_plugin_chain.yml

process:
  title:
    - plugin: concat
      source:
        - source_first_name
        - source_last_name
      delimiter: ' '
    - plugin: callback
      callable: strtoupper

process_plugin_chain_generic_configuration.yml

process:
  destination_field:
    - plugin: plugin_name
      source: source_column_name
      config_1: value_1
      config_2: value_2
    - plugin: plugin_name
      config_1: value_1
      config_2: value_2
    - plugin: plugin_name
      config_1: value_1
      config_2: value_2

process_plugin_deltas_with_subfields.yml

process:
  field_online_profiles/0/uri: source_drupal_profile
  field_online_profiles/0/title:
    plugin: default_value
    default_value: 'Drupal.org profile'
  field_online_profiles/1/uri: source_gitlab_profile
  field_online_profiles/1/title:
    plugin: default_value
    default_value: 'GitLab profile'
  field_online_profiles/2/uri: source_github_profile
  field_online_profiles/2/title:
    plugin: default_value
    default_value: 'GitHub profile'

process_plugin_deltas_with_subfields_generic_configuration.yml

process:
  destination_field/0/subfield_1:
    plugin: plugin_name
    config_1: value_1
    config_2: value_2
  destination_field/0/subfield_2:
    plugin: plugin_name
    config_1: value_1
    config_2: value_2
  destination_field/1/subfield_1:
    plugin: plugin_name
    config_1: value_1
    config_2: value_2
  destination_field/1/subfield_2:
    plugin: plugin_name
    config_1: value_1
    config_2: value_2

process_plugin_deltas_without_subfields.yml

process:
  field_online_profiles/0: source_drupal_profile
  field_online_profiles/1: source_gitlab_profile
  field_online_profiles/2: source_github_profile

process_plugin_explicit_get.yml

process:
  field_online_profiles:
    plugin: get
    source: source_drupal_profile

process_plugin_generic_configuration.yml

process:
  destination_field:
    plugin: plugin_name
    config_1: value_1
    config_2: value_2
    config_3: value_3

process_plugin_implicit_get.yml

process:
  field_online_profiles: source_drupal_profile

process_plugin_subfields.yml

process:
  field_online_profiles/uri: source_drupal_profile
  field_online_profiles/title:
    plugin: default_value
    default_value: 'Drupal.org profile'

process_plugin_subfields_generic_configuration.yml

process:
  destination_field/subfield_1:
    plugin: plugin_name
    config_1: value_1
    config_2: value_2
  destination_field/subfield_2:
    plugin: plugin_name
    config_1: value_1
    config_2: value_2

pseudofields.yml

source:
  constant:
    LINK_TITLE: 'Online profile'
process:
  pseudo_link_text:
    - plugin: get
      source: constant/LINK_TITLE
    - plugin: callback
      callable: strtoupper
  field_online_profiles/0/uri: source_drupal_profile
  field_online_profiles/0/title: '@pseudo_link_text'
  field_online_profiles/1/uri: source_gitlab_profile
  field_online_profiles/1/title: '@pseudo_link_text'
  field_online_profiles/2/uri: source_github_profile
  field_online_profiles/2/title: '@pseudo_link_text'
  field_primary_profile: '@field_online_profiles/0'

source_constants.yml

source:
  constant:
    DRUPAL_LINK_TITLE: 'Drupal.org profile'
    GITLAB_LINK_TITLE: 'GitLab profile'
    GITHUB_LINK_TITLE: 'GitHub profile'
process:
  field_online_profiles/0/uri: source_drupal_profile
  field_online_profiles/0/title: constant/DRUPAL_LINK_TITLE
  field_online_profiles/1/uri: source_gitlab_profile
  field_online_profiles/1/title: constant/GITLAB_LINK_TITLE
  field_online_profiles/2/uri: source_github_profile
  field_online_profiles/2/title: constant/GITHUB_LINK_TITLE