How do I create a View of different entity types?
Views uses a term it calls base table translated into SQL it is the table used for the initial query

SELECT something FROM {base_table};


The thing is that what you want to do is a bit atypical, instead of doing

SELECT something, field_item FROM {entity} LEFT JOIN {field};


You want to do

SELECT something, field_item FROM {field} LEFT JOIN {entity};


In order to do this you need to use the field you are after as the base table. By default this is not possible, but it's not much work required in order to do it. What you need to do, is to use hook_views_data or hook_views_data_alter and add the data for the field table and make it selectable as base table displayed when you create a view.

There's a lot of doc on how to do this (the Views doc), and I have written this in one or more answers here. So I won't bother explaining how to do this.