Entity field value inherited from another entity programmatically?
The way I would tackle this is to create a new field type called “parent_value”.Design it in such a way that it doesn’t actually store any data in the database but instead uses the existing object’s entity reference value to look up the value from the parent. This way you don’t have to rely on any sort of Dripal trigger to alter the value in the child, which will ensure that if values are updated programically, they will still match.

There is also the added advantage that creating a custom field will teach you everything you need to know about Drupal 8 plug-ins.