How do I prevent unauthorized access to my personal accounts?

Not sharing and not reusing passwords and passcodes, and enabling and using two-factor authentication, and not sharing an Apple Account, as a starting point.

Some background on what is happening in your particular case?

You make sure your passwords to your accounts aren't known by anyone other than you.

No one can access your Apple Account unless you have provided them with the log on details to do so.

Have you given out the name of your Apple Account.....(previously called Apple ID).....and the Password to that account to anyone?

If yes, do you trust them with your personal data? Do you also trust them to not provide that log on information to anyone else?