https://understanddrupal.com/articles/drupal-migrations-reference-list-configuration-options-source-plugins

content_entity.yml

source:
  plugin: content_entity:node
  bundle: article
  include_translations: false

csv.yml

source:
  plugin: csv
  path: modules/custom/ud_migrations/ud_migrations_csv_source/sources/udm_photos.csv
  ids: [photo_id]
  header_offset: null
  fields:
    - name: photo_id
      label: 'Photo ID'
    - name: photo_url
      label: 'Photo URL'

d8_config.yml

source:
  plugin: d8_config
  key: migrate
  skip_count: true

embedded_data.yml

source:
  plugin: embedded_data
  data_rows:
    -
      unique_id: 1
      creative_title: 'The versatility of Drupal fields'
      engaging_content: 'Fields are Drupal''s atomic data storage mechanism...'
    -
      unique_id: 2
      creative_title: 'What is a view in Drupal? How do they work?'
      engaging_content: 'In Drupal, a view is a listing of information. It can a list of nodes, users, comments, taxonomy terms, files, etc...'
  ids:
    unique_id:
      type: integer

empty.yml

source:
  plugin: empty
  constants:
    entity_type: node
    field_name: body

migrate_source_plugin_definitions.sh

# List of migrate source plugin definitions.
$ drush php:eval "print_r(\Drupal::service('plugin.manager.migrate.source')->getDefinitions());"

# List of migrate source plugin ids.
$ drush php:eval "print_r(array_keys(\Drupal::service('plugin.manager.migrate.source')->getDefinitions()));"

settings.php

<?php

$databases['default']['default'] = [
  'database'  => 'drupal-8-or-9-database-name',
  'username'  => 'drupal-8-or-9-database-username',
  'password'  => 'drupal-8-or-9-database-password',
  'host'      => 'drupal-8-or-9-database-server',
  'port'      => '3306',
  'namespace' => 'Drupal\\Core\\Database\\Driver\\mysql',
  'driver'    => 'mysql',
];

$databases['migrate']['default'] = [
  'database'  => 'drupal-6-or-7-database-name',
  'username'  => 'drupal-6-or-7-database-username',
  'password'  => 'drupal-6-or-7-database-password',
  'host'      => 'drupal-6-or-7-database-server',
  'port'      => '3306',
  'namespace' => 'Drupal\\Core\\Database\\Driver\\mysql',
  'driver'    => 'mysql',
];

spreadsheet.yml

source:
  plugin: spreadsheet
  file: modules/custom/ud_migrations/ud_migrations_sheets_sources/sources/udm_book_paragraph.ods
  worksheet: 'UD Example Sheet'
  header_row: 1
  origin: A2
  columns:
    - book_id
    - book_title
    - 'Book author'
  row_index_column: 'Document Row Index'
  keys:
    book_id:
      type: string

url.yml

source:
  plugin: url
  data_fetcher_plugin: file
  data_parser_plugin: json
  urls:
    - modules/custom/ud_migrations/ud_migrations_json_source/sources/udm_data.json
  item_selector: /data/udm_people
  fields:
    - name: src_unique_id
      label: 'Unique ID'
      selector: unique_id
    - name: src_name
      label: 'Name'
      selector: name
    - name: src_photo_file
      label: 'Photo ID'
      selector: photo_file
    - name: src_book_ref
      label: 'Book paragraph ID'
      selector: book_ref
  ids:
    src_unique_id:
      type: integer