Where are comments coded when using custom theme?

Generally comments are being displayed using comments.php, the template is rendered using a function called comments_template();.

If-

• there is a comment template in your theme
• and it contains a function called comment_form(),
• and the comment template is being displayed below the post (in single.php, page.php etc.),
• and your comment settings from the Admin panel is open,

then the comment should show up.

This is how usually comments work in a theme templates.

If still you can’t see any comments, then you have to debug it yourself. The common debugging process consists:

• Setting WP_DEBUG to true
• Deactivating all the plugins
• Switching back to any Default theme

Then observe what happens. If the problem still resides, then I’m afraid I can’t help from here. But if it solves then you can think the theme or any plugin is doing something wrong.