Schema API

Defining a proper Schema for a custom field Type in Drupal 8+ doesn't seem to be very well documented yet. Luckily, the core gives us some good examples to start from. For that reason, you should have a look into the core modules that

As heavily discucced in this years old issue: [#937442] there isn't a good solution or helper in Drupal core currently to change the schema of a custom field. Modules like field_collection dynamic_entity_reference (#51) address