Entity reference field dependent on entity reference in referenced entity of another entity reference field
This isn't easily possible at the moment with the standard widget/autocomplete API as it has no context from where it is referenced.

To get something like that, you need a custom widget, one that supports to pass an argument to the views query based on your current node and then you need to connect it with your other field so that setting/changing a value there sends an ajax request to update your other widget, which then needs to get that value and pass it along to the views selection plugin.

Some related core issues:
* https://www.drupal.org/node/2776571
* https://www.drupal.org/node/2280479