I was searching for how to find field info and came across this post: getFieldDefinitions on Drupal 8 (why all these deprecated functions on DP8?).
Which gives an answer of:
but my question is generically how would a developer tell from this page on the Drupal API docs: https://api.drupal.org/api/drupal/core%21lib%21Drupal%21Core%21Entity%21EntityFieldManager.php/function/EntityFieldManager%3A%3AgetFieldDefinitions/8.2.x that this needs to be called as a service and that the service is entity_field.manager?
.. and, why does simply: