Account
You can find the email address linked to the account in the Account menu in the left side menu of the App, in File > Account in the macOS app, and in the Settings menu of the Web App (gear icon at the bottom right).
How to open "Account" menu in the macOS app
Why have I this odd @privaterelay.appleid.com email address?
The account you are using is linked to an anonymous Apple email address created automatically for you by Apple when you signed in with Apple. For more info check this page https://support.apple.com/HT210318
I'm no longer logged in and I don't remember the email address I used
Please check your email inbox looking for an email with the subject “Welcome to Moleskine Journey”, this email has been sent to you when you registered to the service, check the email bottom lines, it should report your sign-in email address.
Subscription
You can check your account status in the side menu, opening the “Account” menu, or, if you subscribed through the App Store, using the Apple page for the subscriptions management.
How to unsubscribe
You can manage your subscription from the "Account" menu. From there you can tap the "unsubscribe" button.
Alternatively you can check the subscription status of the app using the Apple page for the subscriptions management.
If you unsubscribed before the free trial period end you’ll not be charged (on iOS cancel at least 24h before the end of the trial as Apple advise to be sure to not be charged).