Entity reference and associated data Assuming I have two distinct entities A and B, with B having an Entity Reference field to A, how can I associate other data (=other fields) with the Entity Reference?

For example, assume that A is "Political parties" and B "Politicians", and I'd like to store a date range field for the politician's membership in said party.
You can create a paragraph type (paragraph module replaces field collection in Drupal 8) which contains a reference to your entity type A and a (or more) date and in your entity type B, have a reference to this paragraph.

You can also create a custom field which contains a reference to your entity type A and a date if you want have particular behavior (for example, don't allow users to sort items and sort them automatically by date).