How will the callback work for java spring boot app?

[krs8785]

I have a spring boot + react app to which I am working on adding a shopify integration. I am new to this and what I gathered so far is 

1. I need to first redirect the user to their store using https://{shop}.myshopify.com/admin/oauth/authorize?client_id={client_id}&scope={scopes}&redirect_uri...

Now this will be on the front end. Once they authenticate, they will be redirected to the URL mentioned.

My question is
1. Should the redirect be to an API or to a page inside the app? If its the webpage, How do I validate the redirect in react? If its an API, how will java then redirect user back to the app from shopify? 

I might be wrong but I feel it should be a redirect to the webpage. But as an example lets say the redirect is on the main dasboard page how will I validate the callback?

[BALKALOX]

Have you already find anything about that?
I'm trying to implement a payment service in my springBoot web-store using shopify, but cannot find anything about that.
Maybe you know how to help me?
I mean, I cannot even find maven dependency or something like that to work with api  of shopify payments