How do you find a file in the media library using the file URL?

Based on my research, you would need to run a query on directly on the database using WPDB. So you would do something like this:

Add this in your functions.php file:

// retrieves the attachment ID from the file URL
function get_image_id($image_url) {
    global $wpdb;
    $the_attachment = $wpdb->get_col($wpdb->prepare("SELECT ID FROM $wpdb->posts WHERE guid='%s';", $image_url )); 
    return $the_attachment[0]; 
}

And then you can apply it wherever you like this way:

$attachment_url="http://example.com/wp-content/uploads/2020/10/28/just_for_the_lolz.jpg";
$attachment_id = get_image_id($image_url);

$attachment_id will have the attachment ID so you can do whatever you want with it.