Programmatically, I want to activate / deactivate a plugin of this specific blog/site in a WordPress Multisite. Any help will be gladly appreciated, thank you!
From your Network Dashboard go to any subsite dashboard
Then go to Plugins and activate plugin on this specific blog
In case you want to do it programmatically you can use activate_plugins() or deactivate_plugins() after a switch_to_blog( $blog_id ) function, example:
add_action( 'admin_init', 'true_plugin_off_in_theme' );
function true_plugin_off_in_theme() {
switch_to_blog( 'PASS_BLOG_ID_HERE' );
deactivate_plugins( 'true-instagram-widget/true-instagram-widget.php' );
restore_current_blog();
}