<h1><?= Titles\title(); ?></h1>
<?php if(has_excerpt()): ?>
<?php the_excerpt(); ?>
<?php endif;?>
This is pulling the heading from the title of the page, and the text from the page’s excerpt. The excerpt field didn’t appear in the screenshot you included of the post, but it should be there. It may have been hidden by someone adjusting the Screen Options. You can change that by clicking on the “Screen Options” dropdown in the upper right corner in the admin:
The issues you’re describing seem like they’re related to WordPress, not Sage specifically. I’d strongly recommend reading through the WordPress documentation to get an idea of how themes are structured and how they work: Theme Developer Handbook | WordPress Developer Resources