https://understanddrupal.com/articles/using-process-plugins-data-transformation-drupal-migrations

chaining_process_plugins.yml

id: udm_process_intro
label: 'UD Process Plugins Introduction'
source: ...
process:
  type: ...
  title:
    -
      plugin: concat
      source:
        - first_name
        - last_name
      delimiter: ' '
    -
      plugin: callback
      callable: mb_strtolower
    -
      plugin: callback
      callable: ucwords
destination: ...

concat_plugin.yml

id: udm_process_intro
label: 'UD Process Plugins Introduction'
source:
  plugin: embedded_data
  data_rows:
    -
      unique_id: 1
      first_name: 'FELIX'
      last_name: 'DELATTRE'
    -
      unique_id: 2
      first_name: 'BENJAMIN'
      last_name: 'MELANÇON'
    -
      unique_id: 3
      first_name: 'STEFAN'
      last_name: 'FREUDENBERG'
  ids:
    unique_id:
      type: integer
process:
  type:
    plugin: default_value
    default_value: page
  title:
    plugin: concat
    source:
      - first_name
      - last_name
    delimiter: ' '
destination:
  plugin: 'entity:node'

default_value_plugin.yml

source: ...
process:
  type:
    plugin: default_value
    default_value: page
destination:
  plugin: 'entity:node'

explicit_get_plugin.yml

process:
  title:
    plugin: get
    source: creative_title

general_process_plugin_configuration.yml

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

implicit_get_plugin.yml

process:
  title: creative_title