To get have the fields display on your product pages you’ll need to add the app block in your theme. Here’s how:
From your Shopify admin, go to Online Store > Themes.
Find the theme that you want to edit, and then click Customize.
Open the drop-down menu at the top of the page.
Select Products, and then select the product template that you want to edit from the dropdown menu.
Under the panel click the + Add block or + Add section button depending on where you want the product custom fields to appear.
From the APPS section, click Fields to add the block.
Click Save to activate it.
If you’d like to see each step in action, check out the video below for a quick walkthrough: