Understanding Shopify App Bridge 2 and shopify_app (Rails 6) Auth process

Question

So after following several Shopify's tutorials and docs, I'm trying to migrate my Shopify's embedded app from the cookie sessions to JWT using AppBridge 2.

I'm using AppBridge to setup:

 import { getSessionToken } from "@shopify/app-bridge-utils";

 window.app = createApp({
   apiKey: data.apiKey,
   host: data.host,
   forceRedirect: true
});     

on my FrontEnd JS code, which results in the sessionToken attached to the window:

window.sessionToken = await getSessionToken(app);

until here all works fine, I can load my app in the iframe and we live in a happy world.

However, my Rails controllers (called through Ajax) are sending a 401 Unauthorized message, so apparently, I'm authenticated on the FrontEnd but my silly RoR controllers are not aware of that.

Such controllers (as Shopify's documentation indicate) inherit from the authenticated_controller.rb file:

class AuthenticatedController < ApplicationController
  include ShopifyApp::EnsureAuthenticatedLinks # only JWT
  include ShopifyApp::Authenticated

end

I see the class ShopifyApp::Authenticate (from the shopify_app gem) uses the library:

/lib/shopify_app/controller_concerns/login_protection.rb#L100

with the method:

 def jwt_shopify_domain
   request.env['jwt.shopify_domain']
 end 

so, the JavaScript code in the front-end:

 window.sessionToken = await getSessionToken(app) 

sets the value for 'jwt.shopify_domain' to be taken (read) for the backend code? or from where the value for 'jwt.shopify_domain' comes from?

Also I noticed that the route:

auth_shopify_callback GET  /auth/shopify/callback(.:format)  shopify_app/callback#callback

still exists, how this route is related to AppBridge? Previously I used that route to setup manually the cookie session, should I still use it?

In summary: how the rails code on the backend "knows" that AppBridge got a valid sessionToken in the FrontEnd?