https://understanddrupal.com/articles/understanding-entitylookup-and-entitygenerate-process-plugins-migrate-tools

process_field_tags.yml

field_tags:
  - plugin: skip_on_empty
    source: src_fruit_list
    method: process
    message: 'No src_fruit_list listed.'
  - plugin: explode
    delimiter: ','
  - plugin: callback
    callable: trim
  - plugin: entity_generate
    entity_type: taxonomy_term
    value_key: name
    bundle_key: vid
    bundle: tags

process_uid.yml

uid:
  - plugin: entity_lookup
    entity_type: user
    value_key: name
    source: src_creative_author
  - plugin: default_value
    default_value: 1

udm_nodes_sample.json

{
  "data": {
    "udm_nodes": [
      {
        "unique_id": 1,
        "thoughtful_title": "Amazing recipe",
        "creative_author": "udm_user",
        "fruit_list": "Apple, Pear, Banana"
      },
      {...},
      {...},
      {...}
    ]
  }
}