Cannot find a way to query for all posts where a specific user is a (co)author

[roland-jungwirth]

I need to set up a custom query (for an Elementor loop grid) that constraints the query to all posts from a specific author and/or co-author. Unfortunately I did not find anything in the code. Obviously this won't work

$query->set('author', $author_id);

so how can I achieve this?