https://understanddrupal.com/articles/how-debug-drupal-migrations-part-2

composer.json

"extra": {
  "patches": {
    "drupal/migrate_devel": {
      "drush 9 support": "https://www.drupal.org/files/issues/2018-10-08/migrate_devel-drush9-2938677-6.patch"
    },
    "drupal/migrate_tools": {
      "--limit option": "https://www.drupal.org/files/issues/2019-08-19/3024399-55.patch"
    }
  }
}

debug_plugin.yml

field_tags:
  - plugin: skip_on_empty
    source: src_fruit_list
    method: process
    message: 'No fruit_list listed.'
  - plugin: debug
    label: 'Step 1: Value received from the source plugin: '
  - plugin: explode
    delimiter: ','
  - plugin: debug
    label: 'Step 2: Exploded taxonomy term names '
    multiple: true
  - plugin: callback
    callable: trim
  - plugin: debug
    label: 'Step 3: Trimmed taxonomy term names '
  - plugin: entity_generate
    entity_type: taxonomy_term
    value_key: name
    bundle_key: vid
    bundle: tags
  - plugin: debug
    label: 'Step 4: Generated taxonomy term IDs '

debug_plugin_output.sh

$ drush migrate:import udm_config_entity_lookup_entity_generate_node --limit=1
Step 1: Value received from the source plugin: Apple, Pear, Banana
Step 2: Exploded taxonomy term names Array
(
    [0] => Apple
    [1] =>  Pear
    [2] =>  Banana
)
Step 3: Trimmed taxonomy term names Array
(
    [0] => Apple
    [1] => Pear
    [2] => Banana
)
Step 4: Generated taxonomy term IDs Array
(
    [0] => 2
    [1] => 3
    [2] => 7
)
 [notice] Processed 1 item (1 created, 0 updated, 0 failed, 0 ignored) - done with 'udm_config_entity_lookup_entity_generate_node'

fields_source.sh

$ drush migrate:fields-source udm_csv_source_node
 -------------- ------------- 
  Machine Name   Description  
 -------------- ------------- 
  unique_id      unique_id    
  name           name         
  photo_file     photo_file   
  book_ref       book_ref     
 -------------- -------------

import_image_migrate_debug_output.sh

$ drush migrate:import udm_csv_source_image --migrate-debug --limit=1
┌──────────────────────────────────────────────────────────────────────────────┐
│                                   $Source                                    │
└──────────────────────────────────────────────────────────────────────────────┘
array (10) [
    'photo_id' => string (3) "P01"
    'photo_url' => string (74) "https://agaric.coop/sites/default/files/pictures/picture-15-1421176712.jpg"
    'path' => string (76) "modules/custom/ud_migrations/ud_migrations_csv_source/sources/udm_photos.csv"
    'ids' => array (1) [
        string (8) "photo_id"
    ]
    'header_offset' => NULL
    'fields' => array (2) [
        array (2) [
            'name' => string (8) "photo_id"
            'label' => string (8) "Photo ID"
        ]
        array (2) [
            'name' => string (9) "photo_url"
            'label' => string (9) "Photo URL"
        ]
    ]
    'delimiter' => string (1) ","
    'enclosure' => string (1) """
    'escape' => string (1) "\"
    'plugin' => string (3) "csv"
]
┌──────────────────────────────────────────────────────────────────────────────┐
│                                 $Destination                                 │
└──────────────────────────────────────────────────────────────────────────────┘
array (4) [
    'psf_destination_filename' => string (25) "picture-15-1421176712.jpg"
    'psf_destination_full_path' => string (25) "picture-15-1421176712.jpg"
    'psf_source_image_path' => string (74) "https://agaric.coop/sites/default/files/pictures/picture-15-1421176712.jpg"
    'uri' => string (29) "./picture-15-1421176712_6.jpg"
]
┌──────────────────────────────────────────────────────────────────────────────┐
│                             $DestinationIDValues                             │
└──────────────────────────────────────────────────────────────────────────────┘
array (1) [
    string (1) "3"
]
════════════════════════════════════════════════════════════════════════════════
Called from +56 /var/www/drupalvm/drupal/web/modules/contrib/migrate_devel/src/EventSubscriber/MigrationEventSubscriber.php
 [notice] Processed 1 item (1 created, 0 updated, 0 failed, 0 ignored) - done with 'udm_csv_source_image'

import_migrate_debug.sh

# Import only one element.
$ drush migrate:import udm_csv_source_image --migrate-debug --limit=1

# Use the row's unique identifier to limit which element to import.
$ drush migrate:import udm_csv_source_image --migrate-debug --idlist="P01"

import_paragraph_migrate_debug_output.sh

$ drush migrate:import udm_csv_source_paragraph --migrate-debug --limit=1
┌──────────────────────────────────────────────────────────────────────────────┐
│                                   $Source                                    │
└──────────────────────────────────────────────────────────────────────────────┘
Omitted.
┌──────────────────────────────────────────────────────────────────────────────┐
│                                 $Destination                                 │
└──────────────────────────────────────────────────────────────────────────────┘
array (3) [
    'field_ud_book_paragraph_title' => string (32) "The definitive guide to Drupal 7"
    'field_ud_book_paragraph_author' => string UTF-8 (24) "Benjamin Melançon et al."
    'type' => string (17) "ud_book_paragraph"
]
┌──────────────────────────────────────────────────────────────────────────────┐
│                             $DestinationIDValues                             │
└──────────────────────────────────────────────────────────────────────────────┘
array (2) [
    'id' => string (1) "3"
    'revision_id' => string (1) "7"
]
════════════════════════════════════════════════════════════════════════════════
Called from +56 /var/www/drupalvm/drupal/web/modules/contrib/migrate_devel/src/EventSubscriber/MigrationEventSubscriber.php
 [notice] Processed 1 item (1 created, 0 updated, 0 failed, 0 ignored) - done with 'udm_csv_source_paragraph'