The Entity Admin Handlers module provides an out-of-the box admin UI for custom entity types whose configuration means that the admin UI code in Drupal core doesn't fit.

This reduces the need for boilerplate code to provide a UI to manage fields on the entity type and its bundles.

The following cases are provided for.

Single bundle entity type

This is for entity types that do not use bundles and in effect, have a single
bundle which typically has the same name as the entity type.

This is identical to the way the core user entity type works.