Part 1 is to create the default values for each sub field. This is possible through the normal field edit UI.
Part 2 is to use a filter called acf/load_value. Pleae visit the docs page to read up on this filter. Use this filter to hook in and modify the repeater field’s value. If the value is empty (new post), then return an array. This will trick ACF into rendering the row of data using the default values set earlier.
Does that help?
Welcome to the Advanced Custom Fields community forum.
Browse through ideas, snippets of code, questions and answers between fellow ACF users