To show ACF Repeater inside the ACF Relationship we will use ACF Repeater Widget & ACF Relationship Widget.

In continuous to our Nested Repeater Tutorial, we shared how to use Relationship Inside the Repeater.

In nested repeater, we showed to show lessons nested inside the course repeater field.

Then we added writers from another "Writer" custom post type using ACF relationship inside the repeater. See tutorial here

In this tutorial, we are adding a Books repeater for writers and showing them in ACF relationship.
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Below is our template for ACF Relationship containing ACF Repeater for Books:

Below is our single course page, containing Nested Repeaters and ACF Relationship with Repeater.