Integrate Webform.html.twig into existing template
Conceptually, you shouldn't be thinking about it as including templates in other templates. Other CMS'es are more "template based" so to speak, but Drupal isn't. Building your content types and pages in Drupal means telling Drupal (through configuration or in code) how to construct its render arrays. From there on, Drupal will figure out which templates to use and how to merge the resulting HTML of one template into another template.

So that's the theory... now what's the problem in your case? The most obvious reason I can think of, is that you added the field, but did not yet configure it to be displayed. To do so, make sure Field UI module is enabled and then go the "Manage Display" page of your content type. The URL will look like /admin/structure/types/manage/[type]/display where "[type]" is the machine name of your content type. If you see your field in the "disabled" part of the form, drag it upwards to make it enabled and save the form. (This is an example of the "through configuration" method I mentioned earlier.)

If the above does not solve your problem, please add more details to your original question.