The way nested repeaters in a group field stores its subfield values is:
group_name_repeater_name_X_repeater_subfield
Where X is the repeater row and it increments starting at 0 so if you had a field called name in your book videos nested repeater, if any post had a value there, it would be at:
book_videos_bk_videos_0_name
That being said, you could adjust your meta_query to just query if that key exists, you don’t need to check if the group exists, as well. You’d just grab every post that has something in the first repeater spot for any required subfield value of the repeater. Does that make sense?
