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 a 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