WordPress provide the reed support with URL. We can see the site feed with below URL pattern:
The /feed/ is generated from the WordPress PHP file wp-includes\feed-rss2.php
The have_posts() loop is executed while generating the feed. So, We can use the filter
pre_get_posts to show our custom post type feed.
Below is the sample code which add the post, astra-portfolio, and deal post types into the feed.