What is the difference between core.entity_form_mode and core.entity_form_display?
entity_form_mode would be the form mode (/admin/structure/display-modes/form)


37 - Form mode foo available for use on node entities (shows as
content on the link above) 
38 - Form mode register available on
user entities.


entity_form_display is the configuration for that mode on a specific bundle - how the fields are arranged (/admin/structure/types/manage/book/form-display/{form_mode}) - in your case form_mode would be foo.


36 - Form display settings for the default display on the book bundle (content type) of the node entity.


The reason 36 is default is because you have arranged your form display (/admin/structure/types/manage/book/form-display) but you haven't enabled your foo mode.

At the bottom of that page is a fieldset called "Custom display settings". Expand that and you can enable your foo display for that content type.



In the picture, I have a created a form mode called Inline Entity Form, yours should say foo