How do I find "where-used" / many-to-one related entities?
If you want a quick solution in code, you can use the same way as in drupal 7:

Add a field (configurable in view modes):

hook_entity_extra_field_info

Put the code for the field in here:

hook_ENTITY_TYPE_view

Use this code:

How to check if an article is referenced from a collection?