Hi AminaRAli,
Without knowing the full scope of what’s required, I don’t think the Partner API would be required as this is more applicable to partner activity (partner dashboard, sales of apps/ themes etc). So the Admin API may contain most of what you’re looking to achieve, along with the Subscriptions APIs.
Depending on what is required, you might be able to create one custom app that can handle transferring data between the different platforms you’re working with - the customers and orders objects and queries that are part of the Admin API will likely be important for your use case. You may also need to access the Storefront API for managing how store data is displayed.
Hope this helps!