How to get post from all Blog Multisite to the Main Site?

So, actually there is two ways to do it.

The first one is using switch_to_blog() and restore_current_blog() functions, example:

// in this variable you can pass all the blog IDs you would like to display posts from
$blog_ids = array( 1, 3 );

foreach( $blog_ids as $id ) {

    switch_to_blog( $id );

    $args = array(); // any WP_Query args should be here
    $query = new WP_Query( $args );

    if( $query->have_posts() ) : 
    while( $query->have_posts() : $query->the_post();

        // your post template goes here

    endwhile;
    endif;
    wp_reset_postdata();

    restore_current_blog();

}

But if you do not want to switch between the blogs each time, then I recommend my – paid and restricted access – plugin which indexes all the posts from all your website into another database table and then you can use WP_Query analogue to get all the posts within just one loop. Example of this plugin https://rudrastyh.com/plugins/get-posts-from-all-blogs-in-multisite-network