Page fields and global variables can be used inside code of pages/parts/layouts/snippets as predefined variables:
<h1 class="title">{% page.fields['field_name']%}</h1> <!-- field from current page -->
{% pages['page_name'].fields['field_name'] %} <!-- field from 'page_name' page -->
<p>{% assign new_variable = site.variables['promotion_active'] %}</p> <!-- get global variable value... -->
<b>{{new_variable}}</b> <!-- ...and use it inside code -->