In Multisite – main site blog post should be display in minisite or subsite

I can’t write the whole solution for what you need, but to achieve this you need to add code to your theme using functions like these which help you to figure out which multisite you’re on, and pull a post from a different multisite:

get_current_blog_id() – get current multisite site ID

switch_to_blog(123) – switch to multisite ID 123

restore_current_blog() – if you switched to a different site, switch back to the one you were on before

So if your ‘main site’ is blog ID ‘1’ then you probably need to add some code to your theme that does something like this:

    $mainSite = 1;
    $currentSite = get_current_blog_id();

    if ($currentSite != $mainSite) {
    
        switch_to_blog($mainSite);
 
        // get_posts, WP_Query, etc. to get the posts you want

        // post loop to print the post

        restore_current_blog();
    }