Exactly where I have to insert get_next_post_link() in a post?

When you develop a theme you can add specific styling per each post type. by default they all use the single.php theme file, but single-{post-type}.php will be used for the specific post type being displayed (for the full template hierarchy – https://developer.wordpress.org/themes/basics/template-hierarchy/#the-template-file-hierarchy)

The navigation links like the prev and next ones you are trying to add are usually added outside the loop which displays the content itself. I suggest to take a look at one of the core themes to see what is the canonical way to do it.

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