Uhm, not sure if I’m misunderstanding you but URLs of single posts are simply output with the_permalink() https://codex.wordpress.org/Function_Reference/the_permalink
So, in order to put a link to your single post you’d have:
<a href="<?php the_permalink(); ?>">read more</a>
(of course instead of static “read more” text you could as well put the post title there or whatever)
