Unauthorized access to my iCloud account

Go to Settings > [name on top], and scroll down to remove any device that is not yours. Follow up by changing your Apple Account password.

Change your Apple Account password - Apple Support

To be clear, the only way someone can sign into your account when using Two Factor Authentication is with your Password, AND also the Verification Code sent to your device for anyone that has not previously been allowed to access your account. NEVER share your Apple Account password, and if someone else knows it, you should immediately change it. If you give someone your Password, you are authorizing them to use your account. Also review the information provide here, so you do not inadvertently give out your Password through a phishing message/email.

Recognize and avoid social engineering schemes including phishing messages, phony support calls, and other scams - Apple Support