Get stripe customer id during portal checkout - "ReferenceError: session is not defined"

I've been at this for weeks and have reviewed several different (1) questions (2) on stack around retrieving the customer ID during the stripe checkout process (node / mongoose). Neither question--despite having answers--seem to solve the issue for either the original askers or myself, so giving it another try...

During the signup flow, I'm pushing the user to a stripe portal using stripe's create-checkout-session after the user has created a profile in my database.

I'd then like to take the stripe customer_id and save it to the database so I can monitor the customer's subscription status since I cannot figure out webhooks (I'm a new-ish/intermediate programmer and am conceptually struggling with grasping webhooks).

Create checkout controller function (pushes users to the stripe portal for checkout)

 module.exports.createCheckoutSession = async (req, res) => {
     if (req.isAuthenticated()) {
         try {
             const session = await stripe.checkout.sessions.create({
                 mode: "subscription",
                 line_items: [
                     {
                         price: priceId,
                         quantity: 1,
                     },
                 ],

                 // ?session_id={CHECKOUT_SESSION_ID} means the redirect will have the session ID set as a query param
                 success_url: `${webUrl}/users/${req.user._id}/activations/?session_id={CHECKOUT_SESSION_ID}`,
                 // success_url: `/users/${req.body._id}`,
                 cancel_url: `${webUrl}/canceled.html`,
                 // automatic_tax: { enabled: true }
 
             });
             console.log("customer ID ", session.customer_id);
             console.log("query params ", req.query);
             console.log("req params ", req.params);
             console.log("session", session);
             return res.redirect(303, session.url);

         } catch (e) {
             res.status(400);
             return res.send({
                 error: {
                     message: e.message,
                 }
             });
         }
     }
     req.flash('error', "You must login before changing billing");
     res.render('/login');
 }

My function redirects the user to my 'activations' page as the success url which is where I'm trying to capture the customer_id after it's been created in stripe's checkout but I can't find it. Keeping a lot of the other code out of my activations function, below is how I'm looking for the customer_id:

Activations

module.exports.showActivations = async (req, res) => {
    ...
    console.log("req params ", req.params);
    console.log("req headers ", req.headers)
    console.log("session ", session);
    res.render('users/activations', { activeEvents, qr, qrAgg });
    ...
    }
};

Output of my console.logs

• Req.params only has the user_id from my app when they hit my database to signup (prior to going to the stripe portal)

• Req.headers has some cookies but nothing from stripe

• Session returns 'ReferenceError: session is not defined'

How do I get the customer_id from stripe during the checkout so I can monitor the subscription status?