If you have registered a custom post type and have added a custom template for it to your theme, then you could do something like this.
Add a helper function to functions.php that creates a custom query to find posts matching the topic slug. Using a helper keeps the template file a little cleaner.
// functions.php
function wpse_418573_search_posts_by_topic(string $topic, int $paged = 1) : WP_Query {
return new WP_Query([
'post_type' => 'post',
'post_status' => 'publish',
'posts_per_page' => get_option( 'posts_per_page' ),
's' => $topic,
'paged' => $paged,
]);
}
In your custom template have the default Loop handle the rendering of the CPT’s content. Execute the search query by passing the CPT’s slug as a parameter and loop the found posts to display the search results.
// single-topic.php
<article class="topic">
<section class="topic__description">
<?php while ( have_posts() ): the_post(); ?>
<h1 class="topic__title"><?php the_title(); ?></h1>
<?php the_content(); ?>
<?php endwhile; ?>
</section>
<?php
global $post;
$search = wpse_418573_search_posts_by_topic($post->post_name, get_query_var( 'paged', 1 ));
if ($search->have_posts()): ?>
<section class="topic__search">
<?php while ( $search->have_posts() ): $search->the_post(); ?>
<article class="topic__post entry">
<h2 class="entry__title"><?php the_title(); ?></h2>
<?php the_content(); ?>
</article>
<?php endwhile; ?>
</section>
<?php endif;?>
</article>