Someone is completely able to use my iPhone

If you feel an unauthorized person/app is remotely using, controlling or monitoring your device, then that is possible only if you have done one or more of the following Don'ts...

1. Don't hand over an iPhone to kids or to a stranger without Enabling Guided Access
2. Don't share Apple IDs
3. Don't Jailbreak
4. Don't share sensitive information pertaining to your device
5. Don't give in to Phishing
6. Don't plug in your device in Airports and Public places through third-party cables and trust the device. Beware of Juice Jacking. (Especially in India)
7. Don't leave your iPhone unlocked and unattended in public places like offices, schools, malls, etc.

If one of the above is true then quickly change the Apple ID Password and Return iPhone settings to their defaults.

They would have to have your Apple ID and Password. As long as your iPhone is not jailbroken, there is no way someone could mirror your phone other than setting one up using your Apple ID and Password. Two-Factor authentication is also used to prevent this type of activity.

Two-factor authentication for Apple ID - Apple Support

There are 3 things I would do immediately:

• Change your Apple ID Password
• Verify that you are using Two-factor authentication with trusted devices
• Do a Safety Check on your phone by going to Settings > Privacy & Security > Safety Check.

Hey there,

Why do you think so ? PLEASE provide more details so we can help you better.

This user guide is a personal safety resource for anyone who is concerned about or experiencing technology-enabled abuse, stalking, or harassment. It can help you sever digital ties with those you no longer want to be connected to and outlines the personal safety features that are built into Apple devices.

Learn more about personal safety with Apple devices

Here also you can check if there anyone can access to your device: See who has access to your iPhone or iPad - Apple Support

All the best!

Yet another thing to consider. While is unlikely that your hardware has been compromised, it's possible that your Apple ID has been. See here:

If you think your Apple ID has been compromised - Apple Support