Adding custom field while migrating
field_type_of_content is presumably a term reference field, so the value to set for it would be a term ID, not a term name. If you know the IDs for the destination terms, you can map them directly:

source:
  plugin: d7_node
  node_type: article
process:
  field_type_of_content:
    plugin: default_value
    default_value: 5   # Known D8 ID for the "Article" term


More robustly, you can use migrate_plus's entity_lookup plugin to translate the term name to a term ID:

source:
  plugin: d7_node
  node_type: article
  constants:
    term_name: Article
process:
  field_type_of_content:
    plugin: entity_lookup
    source: constants/term_name
    value_key: name
    entity_type: taxonomy_term
    bundle: type_of_content
    bundle_key: vid