Lost iPhone 6 still control my iCloud
My missing iPhone 6 had my iCloud share on both all my iPhone they’re getting control
iPhone 13 mini, iOS 18
How are "they" getting control?
Was your phone unlocked? Did "they" know your passcode and/or Apple ID password, or trick you into revealing these things with "phishing" messages?
Note that removing your lost iPhone from the list of devices associated with your Apple ID would let anybody who finds the phone reset it and "make your phone their own." But if you think your Apple ID password may have been compromised, you should take steps to secure your account right away.
If you think your Apple Account has been compromised - Apple Support
Find your lost iPhone or iPad - Apple Support
Do's and Don'ts for lost iPhones - Switched Off - Offline - Beware of Phishing - Apple Community