Local json migration: how to set the value of the urls variable?
The data fetcher plugin can't find your JSON file because the format is wrong. The plugin is expecting Drupal's URI format (e.g. public://path/to/data.json or private://path/to/data.json) under its private/public file dir. For example, see the included example migration with the module:

  # The data_parser normally limits the fields passed on to the source plugin
  # to fields configured to be used as part of the migration. To support more
  # dynamic migrations, the JSON data parser supports including the original
  # data for the current row. Simply include the 'include_raw_data' flag set
  # to `true` to enable this. This option is disabled by default to minimize
  # memory footprint for migrations that do not need this capability.
  # include_raw_data: true
  # Flags whether to track changes to incoming data. If TRUE, we will maintain
  # hashed source rows to determine whether incoming data has changed.
  # track_changes: true
  # Copy the example JSON file in artifacts folder to sites/default/files folder.
  urls:
    - 'public://migrate_json_example/products.json'