How do I find "where-used" / many-to-one related entities? A field in my Content Type A references a Content Type B. 

I'd like to find, in each instance of Content Type B, a list of all the As that reference it at a particular point in time. 

[Update]:
I made the question less ambiguous in the light of the answer I found with help from others here.
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?