Find the class responsible for a particular field type (find base field settings)

You want to get exact class for particular plugin field.type, type in this in your devel for "entity_reference"

$field_type_manager = \Drupal::service('plugin.manager.field.field_type');
$class = $field_type_manager->getDefinition('entity_reference')['class'];

this will show you info

Drupal\Core\Field\Plugin\Field\FieldType\EntityReferenceItem

so you can check there settings and code for this field type.
To know what else field types are available and installed, check this post here