Messages for Users

Wordpress has built in .logged-in class name for that. You need to wrap your text in a custom class that is invisible by default and visible for the logged in users.

In your product:
<span class="secretmessageforusers">Bla bla bla</span>

In your CSS:

.secretmessageforusers {display:none;}
.logged-in .secretmessageforusers {display:block;}

Try this snippet:

add_action( 'woocommerce_single_product_summary', 'my_custom_text', 25 );
function my_custom_text() {
  print '<span class="secretmessageforusers">Bla bla bla</span>'.PHP_EOL;
}

I am unable to test it with your theme.

You can use this plugin for snippets:
https://ww.wp.xz.cn/plugins/my-custom-functions/

I tested the code on my site and it worked. I was not able to test it with your theme because it is a commercial theme and I don’t have a copy. Its possible that your theme uses different hooks.

The code I gave you is php and it goes at:
Dashboard > Settings > PHP Inserter
Ensure the “On” button is green on that page.

@superkot gave you some custom css, and this goes at:
Dashboard > Appearance > Customise > Additional CSS

When you have it working, it will show the same text on every product page. Come back for how to have different text for different products.