How to disable the single view / search /archive page for a already-built custom post type?

this custom post type is created by a plugin, I can’t update the
register_post_type code

You can use either the register_post_type_args or register_<post type>_post_type_args hook to filter the post type args like the publicly_queryable in your case. See an example below which uses the former hook, where I also set exclude_from_search to true to exclude posts with the magicai-documents post type from front end search results.

<?php
add_filter( 'register_post_type_args', 'my_filter_post_type_args', 10, 2 );
function my_filter_post_type_args( $args, $post_type ) {
    if ( 'magicai-documents' === $post_type ) {
        $args['publicly_queryable']  = false;
        $args['exclude_from_search'] = true;
    }

    return $args;
}

If the post type should only be made public on the admin side, then you can set the public arg to false, and then set show_ui and show_in_nav_menus to true.

See https://developer.wordpress.org/reference/functions/register_post_type/#parameters.

Alternatively, you could hook on an action like template_redirect and do a conditional tag check like if ( is_singular( 'magicai-documents' ) || is_post_type_archive( 'magicai-documents' ) ) { display a 404 error or (do) something else }, and/or you could filter the WP_Query args using the pre_get_posts action, but changing the appropriate post type args is much easier.

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