A basic Salesforce to Salesforce integration using REST API and OAuth

You can use a connected app to request access to Salesforce data on behalf of an external application. For a connected app to request access, it must be integrated with the Salesforce API using the OAuth 2.0 protocol.

1. Create a connected app in one of the Salesforce instances as below:

You will need to replace the callback URL after the completion of step 2.

2. Create an Auth Provider. Replace the consumer key and secret with the values.

Replace the callback URL in the connected app with the callback URL generated while creating this auth provider.

3. Create a named credential

4. Some Apex code

HttpRequest req = new HttpRequest();
req.setEndpoint('callout:Cross_Org_Report/services/data/v45.0/query?q=select+Email,FirstName,LastName+from+contact');
req.setMethod('GET');
Http http = new Http();
HTTPResponse response = http.send(req);
return response.getBody();