Unauthorized charge on 16th of every month on my bill

First see if you can recognise the charge:

How to see what you were charged for

1. To see your purchase history, go to reportaproblem.apple.com.
2. Sign in with your Apple Account to see a list of purchases.
3. If you want to see family members' purchases, select the Apple Account button and choose a family member.
4. To see the full receipt, including tax, tap or click the date. Multiple purchases, including subscriptions, might be grouped onto one charge.

If the charge is for a subscription that you no longer want, you can cancel the subscription. You can also view your Purchase History on your device.

If you still can't identify the charge

If you checked reportaproblem.apple.com and your emailed receipts and you're still not sure why you were charged, contact Apple Support To see if you can opt in for a refund