Drupal 7 to Drupal 8 migration with configuration entities
The migrate system is stable since Drupal 8.5.0, so this is a great time to review the migration tools provided by the community.
This post does not cover migration from the UI, it focuses on partially customized migration that are runned with Drush. In most cases, you will probably want to review the content model a bit (you know, that content type or field machine name that does not actually reflect the truth since its creation, ...).
The idea behind is to delegate the heavy lifting to Migrate Upgrade for migration template generation, then apply content model changes if needed.