See these security documents, especially the first one.
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/HT204397
UseTwo-factor authentication for Apple ID https://support.apple.com/en-us/HT204915
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And, contact Apple Support:
Get a call from Apple support to help solve your problem using this
link https://getsupport.apple.com/?caller=kbase&PGF=PGF63005&category_id=SC0245&symptom_id=23362
Apple ID Support https://support.apple.com/apple-id
Apple ID account security for country-specific and
region-specific help https://support.apple.com/en-us/HT204169
Get Support: We’re here to help https://getsupport.apple.com/. Go through the options to find the best support option.