Most subscriptions renew automatically after a month or a year, depending on your subscription choice.
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You will be notified on a recurring subscription by Apple - in most cases with an e-mail. Check on your subscription here or contact Apple Support as Apple handles those subscriptions.

You will be notified on a recurring subscription by Google - in most cases, with an e-mail. Check on your subscription here or contact Google Support as Google Play Store handles those subscriptions.