How to add view to view mode? We have a view, custom_view, that was created to be displayed on a page that is of the basic page content type. The basic page content type is using the Default View mode and has a taxonomy term, custom_tag.  

Initially we thought about adding the block to the theme block layout, but that does not provide what we need. The custom_view should only be visible on the basic page content type for the Default View and not others. 

How do you add Views to a View Mode? 

The view has a contextual filter so it takes in the current language and it takes in the value of the custom_tag since it is present on the basic page content type.
How do you add Views to a View Mode?
If you want to add a view to a content type's view mode there is a few ways to do this and depending on your exact requirement one might be better than the others
views field formatter
Use https://www.drupal.org/project/views_field_formatter will allow you to choose a formatter for a field and it will pass automatically the value of the field to the view
viewfield
https://www.drupal.org/project/viewfield will allow you to add a field and to choose what views can be chosen and allows you to also include an argument on the node edit form
EVA: Entity Views Attachment
https://www.drupal.org/project/eva will allow you to create a specific EVA views display and configure it to appear for specific bundles (content types).  After saving the view it will appear as a field in the display mode of that content types so it can be positioned appropriatelly
All the above module can take advantage of arguments and contextual filters so you can do very complex things with them