Fraudulent charges

Find the source in not an easy thing to do, and doing so is essentially a police matter that I can't advise you on. But, I can suggest some security-related documents, that may help you in the future.

If you think your Apple ID has been compromised https://support.apple.com/en-us/HT204145

Security and your Apple ID https://support.apple.com/en-us/HT201303

iCloud security overview  https://support.apple.com/en-us/HT202303

Ways to keep your information safe on Mac https://support.apple.com/en-gb/guide/mac-help/mh11402/mac

Using app-specific passwords https://support.apple.com/en-us/HT201355

UseTwo-factor authentication for Apple ID https://support.apple.com/en-us/HT204915

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Also assuming you already are using two-factor authentication, you can periodically Check your Apple ID device list (https://support.apple.com/en-us/HT205064) and remove any device that shouldn't be signed in using the steps in the link.

If there are none that shouldn’t be signed in, then you should not have a problem at this point. But, if an unauthorized device signed in, Change your Apple ID password https://support.apple.com/en-us/HT201355 

Here is What to do after you change your Apple ID or password https://support.apple.com/en-us/HT204071

See the Apple Support article(s) below.

Apple ID - If you see ‘itunes.com/bill‘ or an unfamiliar charge on your bank, credit card, or debit statement.