How can app modify the content of the front page of the store?

[wintonli883]

I'm learning to develop Shopify's app recently.
Refer to the product review demo : https://github.com/Shopify/product-reviews-sample-app
And build the app according to the documents provided by Shopify

However, I can only see the changes in the app on shop admin page, and can't modify the front-end page display in the Shopify theme,

For example, in the above product reviews demo, I can only create new reviews in the app, but I can't let customers create and view reviews in the product details page of the store.

I haven't figured out how Shopify's app affects the content of the front-end page of the store for a long time. Can anyone give me some advice.