After you've found an app that you want to use, you need to add it to your store. You can either install it from the Shopify App Store or with a third-party install link. If you want to install a custom app, then you need to use the unique install link provided by the developer.
When installing with a third-party install link, you receive a warning that the app isn’t from the Shopify App Store. This doesn’t necessarily mean that the app is unsafe. However, the app has not been through the verification and approval process that’s required for apps that are listed in the Shopify App Store.
If you hire a developer to build a custom app for your Shopify store, then they provide you with an installation link. Custom app installation links are specific to your store, and can't be used to install the app on another store.
For security reasons, custom install links expire after seven days. If
your link expires, then the app developer needs to provide a new one.