Hi SA,
Maybe the following picture would help you to decide how to build new app:

PowerApps is something like a custom tool, to help generate an app which would work with the team. No need to register the Custom App, and the created app (phone or tablet style) could be only managed under PowerApp studio. You may take a look at the article below for more information regarding PowerApps:
Introducting Microsoft PowerApps
While Appsource is used for business users to find and try out line-of-business SaaS apps from Microsoft and Microsoft partners, Apps under Appsource would be built by ISV partners on top of Dynamics, Office, and the Azure platform. Users can also discover content packs and add-ins for Microsoft business app. More information, please see:
Turning business proces into business advantage for organizations everywhere
Also:
Register your app with AppSource
Regards
Community Support Team _ Michael Shao
If this post helps, then please consider Accept it as the solution to help the other members find it more quickly.