After you cancel, they will remain in the subscription list. If there is no cancel button, then they have already been cancelled and will not be renewed. You should cancel at least 24 hours before the renewal date to prevent being charged for the renewal.
If there was a previous subscription that was unable to be processed due to lack of funds, you still need to cancel the subscription and you still owe the amount you were being charged. Cancelling a subscription does not make the charge go away.
You will not be able to download apps or update apps if you currently owe money and if you attempt to do that, you will see the verification screen or one that says there was a problem with your previous purchase. Once you verify your payment method or add a new one that is valid, then that previous charge will be billed to the payment method.
If a message says 'billing problem with previous purchase' or 'verification required' - Apple Support