Why am I being charged and not aware of the bill
I'm being billed and I'm not sure what it's for
iPhone 6 Plus, iOS 11.0.3
You do not say where you see this purchase. If it is an e-mail are you sure it is real and not an attempt to steal your private information ('phishing') (very common)?
Phishing & Other Suspicious Emails - https://www.apple.com/legal/more-resources/phishing/
Identifying legitimate emails from the iTunes Store - http://support.apple.com/kb/HT2075 (the previous link is more specific)
Forward attempts to: reportphishing@apple.com
If you still think the charge is real:
See your purchase history in the iTunes Store on a Mac or PC - http://support.apple.com/HT204088 - Does not include purchases made by others under Family Sharing; those have to be checked from their accounts.
If you see ‘itunes.com/bill‘ or an unfamiliar charge on your bank, credit card, or debit statement - https://support.apple.com/HT201382 - "Before you contact Apple to request a refund for a charge you don't recognize"...