Include JS and CSS into custom page

Hi,

I'm not original developer of the site but trying to solve this problem. I have a custom content type (lets call it my-page) so I was able to set up a Colorbox for Image field within this content type. So normaly when I open /node/1234 which is this type everythng works.

BUT

I have another content type which I can't see/edit under admin interface in Structure -> Content types as it's "generated" by custom module developed by third party and I can't make the Colorbox working there because I don't know how to tell Drupal to load CSS and JS files into that page too.

I can verify on that content type where it's working that both CSS are part of the

themes/<my_theme>/templates/html.html.twig

But don't know how to add that Colorbox items into that custom page too as within the template there are just:

<css-placeholder token="{{ placeholder_token }}">

and

<js-bottom-placeholder token="{{ placeholder_token }}">

I also searched for the placeholder_token variable in all files of my site to find out how/where it's being defined, but haven't found anything what would explain those differencies

Here are examples of resultl code:

common node content:

<script src="/core/modules/history/js/history.js?v=8.7.9"></script>
<script src="/core/modules/history/js/mark-as-read.js?v=8.7.9"></script>
<script src="/libraries/colorbox/jquery.colorbox-min.js?v=8.7.9"></script>
<script src="/modules/colorbox/js/colorbox.js?v=8.7.9"></script>
<script src="/modules/colorbox/styles/plain/colorbox_style.js?v=8.7.9"></script>
<script src="/core/misc/announce.js?v=8.7.9"></script>


  </body>
</html>

<!-- END OUTPUT from 'themes/cav/templates/html.html.twig' -->

and custom_page

<script src="/core/modules/history/js/history.js?v=8.7.9"></script>
<script src="/core/modules/history/js/mark-as-read.js?v=8.7.9"></script>
<script src="/core/misc/announce.js?v=8.7.9"></script>


  </body>
</html>

<!-- END OUTPUT from 'themes/cav/templates/html.html.twig' -->