How to cancel app subscription if it's not listed under my subscriptions?

It is not true that all subscriptions are done through Apple. Many subscriptions are offered by a company directly on their website even though they may have an app. Netflix and YouTube are just 2 obvious examples of apps where a user can have a subscription directly with the company that is not managed by Apple.

In your case though with that specific app, I would suspect it is managed by Apple so it may have been made using a different Apple ID. I recommend 2 approaches to verify this:

• If you received an email that shows the subscription charge, verify the Apple ID that is on that email.
• Go to Settings > [name on top] > Media & Purchases, and verify that the email address there is the same one you used to verify your subscription. Some users, especially long time users, will have a different Apple ID that is used here and after tapping to manage your account you will see additional options including subscriptions.

In addition, if you are the Organizer of a Family Plan the uses Purchase Sharing, then Family Members can use their Apple ID to sign up for subscriptions that will be billed to your payment method. That family member will need to cancel the subscription.

All subscriptions are done through Apple so if it is not listed can you please check if you were charged for it on your credit card?

I am having the same issue. I have Herat it on my Apple Watch and cannot see how to cancel it. I do not see it under apple subscriptions and when I go to the HeartBit app I do not see any way to cancel it.