It works with any theme whose product template accepts app blocks, which covers every current Shopify theme. The widget is a theme app block named Subscription widget that you add inside the product section in the theme editor. It is offered only on product templates, so there is no place for it on a collection or home page.
The app helps you check. The Add subscription widget step of the Setup guide on Home ticks itself off once it finds the block on the default product template of your published theme. It checks that one template and never un-ticks, so treat the tick as a hint and confirm on a real product page. If the block is nowhere in the editor, your theme is older and its product section has no app block support. That step then offers Legacy theme install, which generates a snippet you paste into your theme, and we can place it for you. "Add the subscription widget to your product pages" has the walkthrough.
Stuck on any of this? Open the chat in the bottom right corner of the app and we will help you set it up.
