You can add three different types of apps to your Shopify store: public, custom, and private. You can use Shopify apps to enhance your Shopify admin or add new features.
There are two types of public apps, listed and unlisted. You can install listed apps from the Shopify App Store. Unlisted apps don't appear in Shopify App Store search results or categories. Instead, you can install them from the app's App Store listing or the developer's website.
Both listed and unlisted apps must pass the Shopify App review process.
Custom apps can be used to add features to your Shopify admin or to extend your online store to other platforms using custom storefronts. Custom apps are built exclusively for your Shopify store and aren't listed on the Shopify App Store.
An external app developer can make a custom app for you without needing API access to your store and access to your Shopify admin. You can install a custom app by clicking an install link provided to you by the app's developer.
If you're working with an external app developer, then you need to give them API access to your store and access to your Shopify admin. If you don't feel comfortable giving this access to the developer, then consider asking them to build you a custom app.
Draft apps are used by Shopify Partners during development, and can only be installed on development stores for testing purposes. Installing a draft app on a development store disables store transfer permanently.
Draft apps can only be installed on your store after Shopify review, or once they're converted to a custom app.