Create REST or GRAPHQL API's in Shopify app and use it as third party in other software.

[Faateh]

I am trying to create REST and GRAPHQL API's in the shopify app so that I could use it in my software. What I am trying to do is that whenever someone installs my app and subscribe to my software, I could use it store's data in my software. Now I know that it could happen by creating private app but I don't want to use it. I only want to use public app. Now whenever i refresh shopify app , it's base URL and redirect URL's gets updated due to which I think I cannot use it's API in my software. I am new in using shopify app. Kindly guide me.
Thank you in advance.