Building an App that relies on an external Service

[diycoder]

Hi,

I'm a co-founder and developer of a SaaS product for e-commerce businesses. We have several customers and are looking to create a shopify app. The options I see in the documentation are for a custom app (not listable in the store) or an embedded app (listable in the store).

However, When I search the shopify store there are apps like Klaviyo Email & SMS, Aftership Product Reviews, and many others that do not function like the apps described in the documentation. These apps, take you out of shopify, export some account information, build an account in their external application, log you in, and now your store is connected to their website.  I want that experience for my customers.

I'm looking for documentation or guidance on this.  For all I know it's as simple as building an Embedded app that uses the Shopify Admin API, to then export API Keys and other data to the 3rd party application to allow it access and thus to be managed externally. I've not seen any information on this. 

To summarize, I'm looking for some documentation describing this process, or even someone describing the process and what endpoints data are used to export the data from an embedded app - if that's the process. 

Or maybe you are allowed to list custom Apps in the store??

I already have a lot of business logic built into my application I don't need to replicate it on a Shopify-embedded app. 

Thank you for taking the time to read this post, I hope you can help get me going in the right direction.