Which Microsoft API is utilized in while consuming function acquireTokenOnBehalfOf of SDK "@azure/msal-node"?

Question

We are building a web-based, JavaScript Outlook Add-In which is required to call Microsoft Graph APIs to access the Outlook's user's emails and attachments.

The authentication process is being implemented using Single Sign-On (SSO) as outlined in the Microsoft documentation below:

Documentation Linktext

Snapshot

I am working on integrating the @azure/msal-node SDK into my application to write server-side code, specifically using the ConfidentialClientApplication class to implement the OAuth 2.0 On-Behalf-Of (OBO) flow. My goal is to acquire an access token (let's call it Token B) for accessing downstream APIs like Microsoft Graph by exchanging an existing token (Token A) from client-side code.

My Setup

NodeJS version: 20.12.2

SDK: @azure/msal-node

Class Used: msal.ConfidentialClientApplication

Function: acquireTokenOnBehalfOf

Questions:

1. What Microsoft API is utilized internally by the acquireTokenOnBehalfOfmethod to fetch the token B?
2. Is there any specific resources or documentation links that provide further insights into the OBO flow which helps me getting the Microsoft API used in "acquireTokenOnBehalfOf()" function?