How do I get all the referenced taxonomy terms without the field name?
You can iterate through all the fields on the node and add conditions to check that the field is an entity reference field, and that the target type is taxonomy_term:

// Iterate through each field on the node entity to get all the tagged
// taxonomy terms on the node.
foreach ($entity-&gt;getFields() as $field) {
  // Only look for fields that are entity reference fields.
  if ($field instanceof EntityReferenceFieldItemList) {
    // Get the field settings.
    $field_definition = $field-&gt;getFieldDefinition();
    $target_type = $field_definition-&gt;getSetting('target_type');
    // Check that the field targets are taxonomy terms.
    if ($target_type == 'taxonomy_term') {

      // Do stuff..

    }
  }
}