How can I use View modes for entity without bundles?
tl;dr: If your entity annotation contains an admin form, field UI will work.

This is what they would mark on Quora as 'Questions That Contain Assumptions' and those questions usually contain false assumptions. In this case 'bundle of' doesn't even appear in the field UI directory anywhere. Instead, here's how I'd found the answer to your question -- it's not hard at all. First look in core/modules/field_ui/field_ui.routing.yml. Nothing. Well, it's dynamic, if it's dynamic then it's going to be in core/modules/field_ui/lib/Drupal/field_ui/Routing/ and it is. Let's look in RouteSubscriber.php in that directory:

foreach ($this-&gt;manager-&gt;getDefinitions() as $entity_type_id =&gt; $entity_type) {
  $defaults = array();
  if ($entity_type-&gt;isFieldable() &amp;&amp; $entity_type-&gt;hasLinkTemplate('admin-form')) {


Now a quick counter check on the Node annotation and see, under links, there's "admin-form" = "node.type_edit" so that's that.