Custom Page Template

Hello,

I would like to add a custom about page. I don’t want to use Gutenberg. I want to hardcode (php, custom post types, custom fields, html) all the data inside the template and specifically inside the .entry-content class. I have extended page.php and single.php to page-about.php. How do I populate $post_content with code?

<?php
/**
 * Note to code reviewers: This line doesn't need to be escaped.
 * Function blocksy_single_content() used here escapes the value properly.
 */
echo blocksy_single_content($post_content);
?>