https://understanddrupal.com/articles/migrating-xml-files-drupal

image_migration_process.yml

process:
  psf_destination_filename:
    plugin: callback
    callable: basename
    source: src_photo_url

node_migration_process_destination_and_dependencies.yml

process:
  field_ud_image/target_id:
    plugin: migration_lookup
    migration: udm_xml_source_image
    source: src_photo_file
destination:
  plugin: 'entity:node'
  default_bundle: ud_paragraphs
migration_dependencies:
  required:
    - udm_xml_source_image
    - udm_xml_source_paragraph
  optional: []

paragraph_migration_process.yml

process:
  field_ud_book_paragraph_title: src_book_title
  field_ud_book_paragraph_author: src_book_author

udm_book_paragraph_sample.xml

<?xml version="1.0" encoding="UTF-8" ?>
<data>
  <udm_book_paragraph>
    <book_id>B10</book_id>
    <book_details>
      <title>The definite guide to Drupal 7</title>
      <author>Benjamin Melançon et al.</author>
    </book_details>
  </udm_book_paragraph>
  <udm_book_paragraph>
    ...
  </udm_book_paragraph>
  <udm_book_paragraph>
    ...
  </udm_book_paragraph>
</data>

udm_data_sample.xml

<?xml version="1.0" encoding="UTF-8" ?>
<data>
  <udm_people>
    <unique_id>1</unique_id>
    <name>Michele Metts</name>
    <photo_file>P01</photo_file>
    <book_ref>B10</book_ref>
  </udm_people>
  <udm_people>
    ...
  </udm_people>
  <udm_people>
    ...
  </udm_people>
  <udm_book_paragraph>
    <book_id>B10</book_id>
    <book_details>
      <title>The definite guide to Drupal 7</title>
      <author>Benjamin Melançon et al.</author>
    </book_details>
  </udm_book_paragraph>
  <udm_book_paragraph>
    ...
  </udm_book_paragraph>
  <udm_book_paragraph>
    ...
  </udm_book_paragraph>
  <udm_photos>
    <photo_id>P01</photo_id>
    <photo_url>https://agaric.coop/sites/default/files/pictures/picture-15-1421176712.jpg</photo_url>
    <photo_dimensions>
      <width>240</width>
      <height>351</height>
    </photo_dimensions>
  </udm_photos>
  <udm_photos>
    ...
  </udm_photos>
  <udm_photos>
    ...
  </udm_photos>
</data>

udm_people_sample.xml

<?xml version="1.0" encoding="UTF-8" ?>
<data>
  <udm_people>
    <unique_id>1</unique_id>
    <name>Michele Metts</name>
    <photo_file>P01</photo_file>
    <book_ref>B10</book_ref>
  </udm_people>
  <udm_people>
    ...
  </udm_people>
  <udm_people>
    ...
  </udm_people>
</data>

udm_photos_sample.xml

<?xml version="1.0" encoding="UTF-8" ?>
<data>
  <udm_photos>
    <photo_id>P01</photo_id>
    <photo_url>https://agaric.coop/sites/default/files/pictures/picture-15-1421176712.jpg</photo_url>
    <photo_dimensions>
      <width>240</width>
      <height>351</height>
    </photo_dimensions>
  </udm_photos>
  <udm_photos>
    ...
  </udm_photos>
  <udm_photos>
    ...
  </udm_photos>
</data>

udm_xml_source_image_source.yml

source:
  plugin: url
  # This configuration is ignored by the 'xml' data parser plugin.
  # It only has effect when using the 'simple_xml' data parser plugin.
  data_fetcher_plugin: file
  # Set to 'xml' to use XMLReader https://www.php.net/manual/en/book.xmlreader.php
  # Set to 'simple_xml' to use SimpleXML https://www.php.net/manual/en/ref.simplexml.php
  data_parser_plugin: xml
  urls:
    - modules/custom/ud_migrations/ud_migrations_xml_source/sources/udm_data.xml
  # XPath expression. It is common that it starts with a slash (/).
  item_selector: /data/udm_photos
  fields:
    - name: src_photo_id
      label: 'Photo ID'
      selector: photo_id
    - name: src_photo_url
      label: 'Photo URL'
      selector: photo_url
  ids:
    src_photo_id:
      type: string

udm_xml_source_node_local_source.yml

source:
  plugin: url
  # This configuration is ignored by the 'xml' data parser plugin.
  # It only has effect when using the 'simple_xml' data parser plugin.
  data_fetcher_plugin: file
  # Set to 'xml' to use XMLReader https://www.php.net/manual/en/book.xmlreader.php
  # Set to 'simple_xml' to use SimpleXML https://www.php.net/manual/en/ref.simplexml.php
  data_parser_plugin: xml
  urls:
    - modules/custom/ud_migrations/ud_migrations_xml_source/sources/udm_data.xml
  # XPath expression. It is common that it starts with a slash (/).
  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

udm_xml_source_node_remote_source.yml

source:
  plugin: url
  data_fetcher_plugin: http
  # 'simple_xml' is configured to be able to use the 'http' fetcher.
  data_parser_plugin: simple_xml
  urls:
    - https://sendeyo.com/up/d/478f835718
  item_selector: /data/udm_people
  fields: ...
  ids: ...

udm_xml_source_paragraph_source.yml

source:
  plugin: url
  # This configuration is ignored by the 'xml' data parser plugin.
  # It only has effect when using the 'simple_xml' data parser plugin.
  data_fetcher_plugin: file
  # Set to 'xml' to use XMLReader https://www.php.net/manual/en/book.xmlreader.php
  # Set to 'simple_xml' to use SimpleXML https://www.php.net/manual/en/ref.simplexml.php
  data_parser_plugin: xml
  urls:
    - modules/custom/ud_migrations/ud_migrations_xml_source/sources/udm_data.xml
  # XPath expression. It is common that it starts with a slash (/).
  item_selector: /data/udm_book_paragraph
  fields:
    - name: src_book_id
      label: 'Book ID'
      selector: book_id
    - name: src_book_title
      label: 'Title'
      selector: book_details/title
    - name: src_book_author
      label: 'Author'
      selector: book_details/author
  ids:
    src_book_id:
      type: string