My phone is being mirrored and controlled by someone else. How can I fix this

See these security-related documents, especially the first two for now.

If you think your Apple ID has been compromised https://support.apple.com/en-us/HT204145. From the link, among other things, Change your Apple ID password and choose a strong password.

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

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

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And, if you can, 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 is 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

For Apple Support help with your problem, use:

Get a call from Apple support to help solve your problem  https://getsupport.apple.com/?caller=kbase&PGF=PGF63005&category_id=SC0245&symptom_id=23362

Apple ID Support https://support.apple.com/apple-id

Apple Support Twitter https://twitter.com/AppleSupport

Apple ID account security for country-specific and region-specific

help https://support.apple.com/en-us/HT204169

Your iPhone can't be "mirrored" on a Samsung. If someone had your Apple ID and password, they could log into it on another Apple device, but you'd get a notification of this. And, even if they did that, they couldn't control your phone.

Why do you think this is happening?