How do I get a refund from Apple for an unauthorized subscription renewal?

Do any or all the charges display when you review your recent purchases here,

https://reportaproblem.apple.com/

Charges display per item. Receipts or statements sent by Apple may display multiple items as a single purchase. Purchases generally are taxable and you must take that into consideration. 

If you need to request a refund, instructions are provided at the link below,

If you want to cancel a subscription from Apple - Apple Support

Request a refund for apps or content that you bought from Apple - Apple Support

If additional assistance is required from Apple Support, please use the links below to contact Apple Support. If you’re in the US please call  1-800-MY-APPLE or outside the US, consult the support articles below.

Contact Apple for support and service - Apple Support

that is normal practice for subscriptions through any company to auto renew. Apple even tells you that you need to cancel the trial at least 24 hours before it ends so you are not charged. Apple has made its decision and unfortunately you’re not gonna be refunded because you were properly charged. Take this as a lesson for the future.

It wasn’t arbitrary and you gave the explicit permission when agreeing to the terms of use and when subscribing to the trial. They’ve already told you twice you won’t be refunded or get your money back.