https://understanddrupal.com/articles/migrating-files-and-images-drupal

file_migration_destination_section.yml

destination:
  plugin: 'entity:file'

file_migration_process_section.yml

process:
  uri: source_column_file_uri

file_migration_source_section.yml

source:
  constants:
    SOURCE_DOMAIN: 'https://agaric.coop'
    DRUPAL_FILE_DIRECTORY: 'public://portrait/'
  plugin: embedded_data
  data_rows:
    - photo_id: 'P01'
      photo_url: 'sites/default/files/2018-12/micky-cropped.jpg'
    - photo_id: 'P02'
      photo_url: ''
    - photo_id: 'P03'
      photo_url: 'sites/default/files/pictures/picture-94-1480090110.jpg'
    - photo_id: 'P04'
      photo_url: 'sites/default/files/2019-01/clayton-profile-medium.jpeg'
  ids:
    photo_id:
      type: string

psf_destination_filename_pseudofield.yml

psf_destination_filename:
  - plugin: callback
    callable: basename
    source: photo_url
  - plugin: skip_on_empty
    method: row
    message: 'Cannot import empty image filename.'

psf_destination_full_path_pseudofield.yml

psf_destination_full_path:
  - plugin: concat
    source:
      - constants/DRUPAL_FILE_DIRECTORY
      - '@psf_destination_filename'
  - plugin: urlencode

psf_source_image_path_pseudofield.yml

psf_source_image_path:
  - plugin: concat
    delimiter: '/'
    source:
      - constants/SOURCE_DOMAIN
      - photo_url
  - plugin: urlencode

uri_entity_property.yml

uri:
  plugin: file_copy
  source:
    - '@psf_source_image_path'
    - '@psf_destination_full_path'
  file_exists: 'rename'
  move: FALSE