This will give me all the posts that are not the current one open, that are written by the same author, AND that meet at least one on the categories on the list.
<?php $loop = new WP_Query([ 'post__not_in' => [$current_id], 'category__in' => $categories, 'author' => $author_id, 'posts_per_page' => 3 ]); ?>
But how could I make a query that gives me all the posts that are written by the same author OR meet one of the current categories?
How can I make
author not exclude each other?
Read more here: WordPress – Querying posts by Author OR category