functions.php

<?php
    $blogs = $wpdb->get_results( "
        SELECT
            `blog_id`
        FROM
            " . $wpdb->blogs . "
    " );
    
    foreach( $blogs AS $blog )
    {
        switch_to_blog( $blog->blog_id );
        
        // default template
        echo '
            <option value="' . get_bloginfo( 'wpurl' ) . '">' . get_bloginfo( 'name' ) . '</option>
        ';

        restore_current_blog();
    }