If you are using a Vintage theme or your theme doesn’t let you place app blocks in the product section, you’ll need to add the custom fields code directly to your theme template.
Here’s how to do it:
In your Shopify admin, go to Online Store > Themes.
Find the theme you want to edit and click Actions > Edit code.
Under the Templates header, open the product-template.liquid file.
Decide where on the product page you want the custom fields to appear.
Paste this code where you want the fields to show:
<custom-fields></custom-fields>
Click Save.
This will add the custom fields to your product page even if your theme does not support app blocks.