How to get a customer who installs an application in HubSpot CRM?

Question

CRM

Good evening!
I am creating an integration for HubSpot CRM. On a test client for developers, I install the integration, after selecting an account, the redirection occurs as expected with the get parameter ?code=*******, then I execute a curl request https://api.hubapi.com/oauth/v1/token .
I get access and refresh tokens.

Question: what's next?
1 - How can I get the data of the client who requested the installation of the application in order to store the received keys for him?
This is necessary in order to get an answer to the question, according to the scheme worked out on other CRMs, whether we have an account (on my website) or not.
2 - Where to redirect the client after authorization/registration?
In the card of the test client, on whose behalf I install the integration, it is displayed in the Connected App list (connected registrations), but in the scopes that are declared in the application (contacts, deals, companies), I can’t somehow call the application and I don’t understand how?

Who worked with this crm - please help.