In the Flex theme, you select your default product page template within the Theme Editor (customize) under Theme Settings > Layout > Templates. Keep in mind, the content blocks configured in the theme editor, will show up for all products using that template.
In order to show unique content blocks for the same template, you would need to duplicate the template.
Current Theme Versions (Online Store 2.0)
Theme versions older than Flex v3.0.0 will need to update to the newest version of the theme before proceeding with this guide.
Since all Flex's product page templates are converted to JSON for Online Store 2.0, this template can be easily duplicated through the theme editor.
In the theme editor (Customize), select Products then [+] Create template with the page selector.
Give the second template a Name, then change the Based on setting to scrolling.
Select Create template to complete the duplication.
Assign Templates to Batches of Products with Bulk Editing