drupal migrate source csv
'process' is not recursive - you can't simply embed processing within another plugin in the general case (only iterator has that ability). Rather, plugins (generally) need to be executed in sequence:

field_subject_terms:
  # First step in the processing pipeline.
  -
    plugin: explode
    source: subject_terms
    delimiter: ','
  # Results of the first step are passed through to the second step.
  -
    plugin: entity_generate