a collection of migrations - i this case to make some groups, group_content and content entities from a csv and a json. The json is read twice; once to create the content entities and then again to create the group_content entities joining them to the right group

migrate_plus.migration.csv_migrate_story.yml

uuid: xxx
langcode: en
status: true
dependencies: {  }
_core:
  default_config_hash: wQBGyxU9I0xkvmhVOYPTI8xAPDRmQqHJRGcumgEen0w
id: csv_migrate_story
migration_tags:
  - CSV
migration_group: mt_stories
label: 'CSV file to Travel Stories'
source:
  plugin: csv
  path: 'public://csv/stories.csv'
  header_row_count: 0
  constants:
    uid: 1
    body: 'A Travel Story by '
  keys:
    - mt_id
  column_names:
    -
      id: id
    -
      mt_id: mt_id
    -
      name: name
    -
      email: email
    -
      desc: desc
    -
      url: url
process:
  type:
    plugin: default_value
    default_value: travel_story
  field_mt_id: mt_id
  field_mt_user: name
  field_mt_email: email
  field_mt_url: url
  label: desc
  uid: constants/uid
  field_description:
    plugin: concat
    source:
      - constants/body
      - name
destination:
  plugin: 'entity:group'
migration_dependencies:
  required: {  }
  optional: {  }

migrate_plus.migration.json_travel_story_entry.yml

uuid: xxx
langcode: en
status: true
dependencies: {  }
_core:
  default_config_hash: wQBGyxU9I0xkvmhVOYPTI8xAPDRmQqHJRGcumgEen0w
id: json_travel_story_entry
migration_tags: null
migration_group: mt_stories
label: 'JSON feed of Travel Story Entries'
source:
  plugin: url
  data_fetcher_plugin: http
  data_parser_plugin: json
  urls: 'http://path.to.json'
  constants:
    uid: 1
    divider: ' '
  fields:
    -
      name: entryid
      label: 'Entry ID'
      selector: entryid
    -
      name: mt_id
      label: 'MoveableType ID'
      selector: blogid
    -
      name: title
      label: Title
      selector: title
    -
      name: date
      label: Date
      selector: date
    -
      name: body
      label: 'Body text'
      selector: body
    -
      name: body_more
      label: body_more
      selector: body_more
    -
      name: teaser
      label: 'Excerpt - sometimes empty'
      selector: excerpt
  ids:
    entryid:
      type: integer
process:
  type:
    plugin: default_value
    default_value: travel_story_update
  mt_id: mt_id
  title:
    plugin: default_value
    default_value: Update
    source: title
  field_date_of_update:
    plugin: process_mt_date
    source: date
  body/value:
    plugin: concat
    source:
      - teaser
      - body
      - body_more
  body/format:
    plugin: default_value
    default_value: rich_text
  field_entryid: entryid
  uid: constants/uid
destination:
  plugin: 'entity:node'
migration_dependencies:
  required:
    - csv_migrate_story

migrate_plus.migration.json_travel_story_entry_groupcontent.yml

uuid: xxx
langcode: en
status: true
dependencies: {  }
_core:
  default_config_hash: wQBGyxU9I0xkvmhVOYPTI8xAPDRmQqHJRGcumgEen0w
id: json_travel_story_entry_groupcontent
migration_tags: null
migration_group: mt_stories
label: 'JSON feed of Travel Story Entries - to create groupcontent'
source:
  plugin: url
  data_fetcher_plugin: http
  data_parser_plugin: json
  urls: 'http://path.to.json'
  constants:
    uid: 1
  fields:
    -
      name: entryid
      label: 'Entry ID'
      selector: entryid
    -
      name: mt_id
      label: 'MoveableType ID'
      selector: blogid
    -
      name: title
      label: Title
      selector: title
    -
      name: date
      label: Date
      selector: date
    -
      name: body
      label: 'Body text'
      selector: body
    -
      name: excerpt
      label: 'Exceprt - sometimes empty'
      selector: excerpt
  ids:
    entryid:
      type: integer
process:
  type:
    plugin: default_value
    default_value: group_content_type_040b636e0ec2b
  gid:
    plugin: migration
    migration: csv_migrate_story
    no_stub: true
    source: mt_id
  entity_id:
    plugin: migration
    migration: json_travel_story_entry
    no_stub: true
    source: entryid
  label: title
  uid: constants/uid
destination:
  plugin: 'entity:group_content'
migration_dependencies:
  required:
    - json_travel_story_entry
    - csv_migrate_story