https://understanddrupal.com/articles/using-constants-and-pseudofields-data-placeholders-drupal-migration-process-pipeline

constants_definition_with_alternative_name.yml

source:
  defaults:
    MY_VALUE: 'http://understanddrupal.com'
  plugin: source_plugin_name
  source_plugin_config: source_config_value
process:
  process_destination: defaults/MY_VALUE

example_constants_definition.yml

source:
  constants:
    MY_STRING: 'http://understanddrupal.com'
    MY_INTEGER: 31
    MY_DECIMAL: 3.1415927
    MY_ARRAY:
      - 'dinarcon'
      - 'dinartecc'
  plugin: source_plugin_name
  source_plugin_config_1: source_config_value_1
  source_plugin_config_2: source_config_value_2
process:
  process_destination_1: constants/MY_INTEGER
  process_destination_2:
    plugin: concat
    source: constants/MY_ARRAY
    delimiter: ' '

example_pseudofields_definition.yml

source:
  constants:
    MY_BASE_URL: 'http://understanddrupal.com'
  plugin: source_plugin_name
  source_plugin_config_1: source_config_value_1
  source_plugin_config_2: source_config_value_2
process:
  title: source_column_title
  my_pseudofield_1:
    plugin: concat
    source:
      - constants/MY_BASE_URL
      - source_column_relative_url
    delimiter: '/'
  my_pseudofield_2:
    plugin: urlencode
    source: '@my_pseudofield_1'
  field_link/uri: '@my_pseudofield_2'
  field_link/title: '@title'

image_migration_process_definition.yml

process:
  title: name
  pseudo_image_url:
    plugin: concat
    source:
      - constants/BASE_URL
      - photo_url
    delimiter: '/'
  pseudo_image_description:
    plugin: concat
    source:
      - constants/PHOTO_DESCRIPTION_PREFIX
      - name
    delimiter: ' '
  field_image:
    plugin: image_import
    source: '@pseudo_image_url'
    reuse: TRUE
    alt: '@pseudo_image_description'
    title: '@title'
    width: photo_width
    height: photo_height

image_migration_source_definition.yml

source:
  constants:
    BASE_URL: 'https://agaric.coop'
    PHOTO_DESCRIPTION_PREFIX: 'Photo of'
  plugin: embedded_data
  data_rows:
    -
      unique_id: 1
      name: 'Michele Metts'
      photo_url: 'sites/default/files/2018-12/micky-cropped.jpg'
      photo_width: '587'
      photo_height: '657'