"Is there a UI-controlled method to do this with fields and/or entity reference?".

No, I don't think so.

Especially not when it's just a text field but also not when it's a more structured type of content. That kind of dynamic refresh/selection update requires custom code. You need to implement a module that alters the form with an ajax element that triggers a form rebuild that updates a second select based on what's in the other.

See How to dynamically populate select options? and How to fill select options based on previous select? for related questions. The first is for Drupal 7 but conceptually, it's still basically the same in Drupal 8.

Explaining everything in detail is too complex for a Drupal Answers Q&A format, you'll need to get started with tutorials on creating a module, form system, ajax and so on and then ask specific detail questions if you're stuck.

Is there a UI-controlled method to do this with fields and/or entity reference?

No, I don't think so.

Especially not when it's just a text field but also not when it's a more structured type of content. That kind of dynamic refresh/selection update requires custom code. You need to implement a module that alters the form with an ajax element that triggers a form rebuild that updates a second select based on what's in the other.

See How to dynamically populate select options? and How to fill select options based on previous select? for related questions. The first is for Drupal 7 but conceptually, it's still basically the same in Drupal 8.

Explaining everything in detail is too complex for the format of this site. You'll need to get started with tutorials on creating a module, form system, AJAX and so on, and then ask specific questions if you're stuck.