https://understanddrupal.com/articles/migrating-data-drupal-subfields

body_subfields.yml

process:
  body/value: profile
  body/format:
    plugin: default_value
    default_value: restricted_html

explicit_subfield.yml

process:
  field_string/value: source_value_string
  field_integer/value: source_value_integer

field_image_subfields.yml

process:
  field_image/target_id:
    plugin: file_import
    source: photo_url
    reuse: TRUE
    id_only: TRUE
  field_image/alt: photo_description
  field_image/title: photo_description
  field_image/width: photo_width
  field_image/height: photo_height

implicit_subfield.yml

process:
  field_string: source_value_string
  field_integer: source_value_integer

source_snippet.yml

source:
  plugin: embedded_data
  data_rows:
    -
      unique_id: 1
      name: 'Michele Metts'
      profile: '<a href="https://www.drupal.org/u/freescholar" title="Michele on Drupal.org">freescholar</a> on Drupal.org'
      photo_url: 'https://agaric.coop/sites/default/files/2018-12/micky-cropped.jpg'
      photo_description: 'Photo of Michele Metts'
      photo_width: '587'
      photo_height: '657'

text_with_summary_subfields.yml

process:
  field_text_with_summary/value: source_value
  field_text_with_summary/summary: source_summary
  field_text_with_summary/format: source_format

In text_with_summary_subfields.yml , I think you want to replace field_text_with_summay with field_text_with_summary (on three lines).

In text_with_summary_subfields.yml , I think you want to replace field_text_with_summay with field_text_with_summary (on three lines).

Thanks for catching that @benjifisher It has been fixed.