Get the custom fields created for the user entity in Drupal8

I was trying to get the custom fields created for the user entity in Drupal8:

Drupal :: service (& # 39; entity_field.manager & # 39;) -> getFieldStorageDefinitions (& # 39; user & # 39;) return all fields

Drupal :: service ('entity_field.manager') -> getFieldDefinitions ('user', & # 39; & # 39;) returns only the default fields

I was doing a diff of the 2 paintings, but I was wondering if there was an appropriate way to achieve it?