Migrate Source CSV is the contrib functionality for migrating CSV files.
Example
The migrate_source_csv_test module in the tests/modules folder provides a fully functional and runnable example migration scenario demonstrating the basic concepts and most common techniques for CSV-based migrations.
To enable test modules, add $settings['extension_discovery_scan_tests'] = TRUE; to your settings.php, or enable the default local.settings.php file that comes with Drupal 8.
See https://cgit.drupalcode.org/drupal/tree/sites/example.settings.local.php
Installing the demo/test module
Enable the module, check status and run a migration: drush en migrate_source_csv_test -y drush migrate-status drush migrate-import migrate_csv_test