Migrating Content from Drupal 8/9 to Drupal 9 - Files

content_entity plugin only works if migrating site within an existing Drupal 8/9 site. If you need to use an external database from a D8/D9 site, you'll need the https://www.drupal.org/project/migrate_drupal_d8 module

Here's an example of migrating files (prior to migrating media items)

id: im_files
label: Migrate public files from legacy D9 to new D9
langcode: en
status: true
dependencies: { }
migration_tags:
  - files

source:
  plugin: d8_entity
  scheme: public
  key: migrate
  entity_type: file

  constants:
    SOURCE_DOMAIN: 'https://legacy-site.dev'
    DRUPAL_FILE_DIRECTORY: 'public://migrated_files/'
process:
  filename:
    -
      plugin: get
      source: filename
  source_public_absolute_path:
    -
      plugin: str_replace
      source: uri
      search: 'public://'
      replace: 'sites/default/files/'
  source_full_path:
    -
      plugin: concat
      delimiter: /
      source:
        - constants/SOURCE_DOMAIN
        - '@source_public_absolute_path'
    -
      plugin: urlencode
  destination_filename:
    - plugin: callback
      callable: basename
      source: filename
    - plugin: skip_on_empty
      method: row
      message: 'Cannot import empty filename.'

  destination_path:
    -
      plugin: concat
      source:
        - constants/DRUPAL_FILE_DIRECTORY
        - '@destination_filename'
    -
      plugin: urlencode

  uri:
    -
      plugin: skip_on_empty
      method: row
      source: '@source_full_path'
      message: 'File field_name does not exist'
    -
      plugin: file_copy
      source:
        - '@source_full_path'
        - '@destination_path'      
  filemime:
    -
      plugin: get
      source: filemime
  filesize:
    -
      plugin: get
      source: filesize
  status:
    -
      plugin: get
      source: status
  created:
    -
      plugin: get
      source: timestamp
  changed:
    -
      plugin: get
      source: timestamp
  uid:
    -
      plugin: default_value
      default_value: 1
destination:
  plugin: 'entity:file'
migration_dependencies: null