Migrate row data into text formatted long multiple
There is a plugin named iterator which allows iterate a list of associative fields to a compound field.
Example:
field_formatted:
      plugin: iterator
      source:
        - fieldA
        - fieldB
      process:
        value: value
        format:
          plugin: default_value
          default_value: 'full_html'

In this case the fields should have this structure
    [
      'fieldA' =&gt; [
         'value' =&gt; 'Chicken',
      ],
    ]

Can you fields you adapt your source fields to use iterator plugin in this way?