The app does not change your prices. If you uninstall the app or disable your promotions, your shop will use standard pricing again. The app allows you to easily display discounted prices throughout your store without having to make any changes to your product catalog! This is achieved through Dynamic Pricing, which requires setup on your theme. Learn more here.

It also does not add metadata to your products. All the logic is handled through the promotions, we store no information in your product catalog.

The app leverages the existing Price (not the Compare at price) you have set in your product catalog:

The app allows for discounting specific collections or excluding collections of products from your promotions. Using Shopify's collections you can easily build a dynamically updated list of products that have a specific tag, vendor or product type.
Collections can only be used in Discount Ninja if they are published on the Online Store channel. Additionally, if you are using Discount Ninja's sales channel, the products and collections need to be published on that channel as well.

Discount Ninja applies the discounts you define in the app to the cart and checkout.
The price rules can be applied in three ways using triggers:

Discount Ninja can only apply discounts in the context of the Online Store channel. I.e. your webshop. The app is not able to apply discounted prices on:

Discount Ninja displays prices dynamically in the cart, collection, and product pages. It calculates the discounted prices for a product based on the context. This allows you to show a different price when retargeting a visitor who did not complete their purchase. Or a different price to loyal customers, or a discount when visitors spend a specific amount.
To achieve this the app needs some markers to be set up in your theme to allow the app to understand where prices are displayed. The modifications required to your theme are listed here.

On top of that the app can apply one or more discount codes at the cart, using the custom promotion code field, and optionally one more at checkout. This can be useful if you want customers to be able to take advantage of Shopify discount codes that are generated by other apps while still applying Discount Ninja promotions to the cart. Learn more about this here.

The app uses the standard Shopify checkout, but applies discounts to the cart items before the checkout is loaded. It uses Draft Orders (or similar) to achieve this. As you may know, a checkout process that uses a Draft Order is different in two ways due to limitations of the Shopify platform: 1/ it does not have a "return to cart" link (customers have to click the back button of their browser) and 2/ the checkout does not include a discount field (gift cards are allowed though). Discount Ninja provides a solution for the second problem: it is able to show a discount field at checkout when Discount Ninja promotions are applied to the cart. Learn more about this here.

Note that your Abandoned Checkout email flows by default will try to recover an abandoned checkout by sending customers back to the checkout url. That url contains the discounts you provided. As a result, customers that receive such and email are able to check out with discounts applied even if your Discount Ninja promotions are expired or disabled.

Interested? Also, check out the limitations of the app, listed here.