Search to include external content

For searching among your posts through Advanced custom fields, use fallowing query:

$args = [
    'post_type'     => 'post',
    'meta_query'    => [
        'relation'      => 'OR',
        [
            'key'       => 'NAME_OF_ACF_FIELD',
            'compare'   => 'like',
            'value'     => '%'.$search_value.'',
        ]
    ]
];

$items = new WP_Query($args);

while($items->have_posts()) {
    $items->the_post();
    the_title();
}

This will output the titles that matches our search query.
even you can add more items to meta_query array to search for example 3 field together.

Also, you can use WordPress filter hook to modify query before execution
This is an example of modifying where statement on query

add_filter( 'posts_where', 'my_posts_where', 10, 2 );
function my_posts_where( $where, &$wp_query )
{
    global $wpdb;
    if ( $search_title= $wp_query->get( 'search_title' ) ) {
        $where .= ' AND ' . $wpdb->posts . '.post_title LIKE \'' . esc_sql( $wpdb->esc_like( $search_title) ) . '%\'';
    }
    return $where;
}