How does this code enable the redirection flow?

I have a frontend www.myfrontend.tech and a backend www.mybackend.io. My previous developer managed to realize third-party authentications (like Microsoft) with PassportJS. But when looking at his code, I could not understand the flow and how the code enables this. Here is the code:

In the .conf of nginx of www.myfrontend.tech:

upstream mybackend {
   server www.mybackend.io:443;
}

server {
    location ~ /socialLoginSuccess {                                                                                            
        rewrite ^ '/#/socialLoginSuccess' redirect;
    }

    location ~ /auth/(.*) {                                                                                            
        proxy_pass  https://mybackend/front/auth/$1?$query_string;
        proxy_set_header Host www.myfrontend.tech;
    }
    ... ...
}

In frontend/src/router.tsx of www.myfrontend.tech in ReactJS:

function RouterConfig({ history }: any) {
  return (
    <ConnectedRouter history={history}>
      <Layout>
        <Switch>
          <Route path="/socialLoginSuccess">
            <SocialLoginSuccess />
          </Route>
          ... ...

In app.js of www.mybackend.io in NodeJS+ExpressJS+PassportJS:

var _front = require('./routes/front');
app.use('/front', _front);

In routes/front.js of www.mybackend.io:

router.get('/auth/microsoft', passport.authenticate('microsoft', { scope: ['User.Read'] }));
router.get('/auth/microsoft/callback', passport.authenticate('microsoft', {
  successRedirect: '/auth/signinSuccess',
  failureRedirect: '/auth/signinFailure',
  failureFlash: true
}))

router.get('/auth/signinSuccess', function (req, res, next) {
  res.redirect("/socialLoginSuccess");
})

In passport.module.js:

var Microsoft = (function() {
  function Microsoft() {}
  Microsoft.prototype.enable = function() {
    var MicrosoftStrategy = require("passport-microsoft").Strategy;
    passport.use(
      "microsoft",
      new MicrosoftStrategy(
        {
          clientID: keys.MICROSOFT_CLIENT_ID,
          clientSecret: keys.MICROSOFT_CLIENT_SECRET,
          callbackURL: `/auth/microsoft/callback`
        },
        this.loginSuccessfully
      )
    );
  };
  return Microsoft;
})();

During tests of this authentication, I can see in the address bar www.myfrontend.tech/auth/google and www.myfrontend.tech/#/socialLoginSuccess after signing in.

My question is, for example, where successRedirect: '/auth/signinSuccess' goes (backend or frontend)? and where does res.redirect("/socialLoginSuccess") go? How was it enabled by the code?