How can I tell if my iPhone has been compromised?

Here are some things you can do:

Gain control of your Apple Account

1. Change your Apple Account password. Make sure that you use a strong and unique password.
2. If you can't change your Apple Account password because it's already been changed by someone else, reset your password.
3. Go to account.apple.com to update any of personal or security information that isn't correct or that you don't recognize.
4. On account.apple.com, select Devices and remove any devices associated with your Apple Account that you don't recognize.
5. Check with your email provider and cellular carrier to make sure that you control every email address and phone number associated with your Apple Account. For example, check with your cellular carrier that SMS forwarding hasn't been set up for a phone number associated with your Apple Account.

This was copied from the link below, which also offers more tips:

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

ok but that doesn't answer my question on how to get it to stop.

Run Safety Check on the phone.......Safety Check for an iPhone with iOS 16 or later - Apple Support

If still no luck, your next options would be to visit a security specialist....who will likely advise you to erase the phone......or, if you don't want to pay for a security specialist to advise you to erase the phone....erase the phone yourself, so you can set up a new Apple ID and Password

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.

Keep the iPhone updated to the latest iOS always and never Jailbreak. That's it.

iOS / iPadOS devices cannot be hacked or infected with Virus / Malware / Spyware *** unless you have intentionally downloaded spurious software or unauthorized apps directly from the internet and installed them on your device or/and have Jailbroken. 

It (Hacking) also depends on how careful you are in sharing sensitive and valuable information pertaining to your iPhone such as Passcode, Password, etc with your friends and family members.

Be judicious when sharing the device's sensitive and valuable information with friends and family members.

**The primary reason for this is Sandboxing. All third-party apps are “sandboxed”, so they are restricted from accessing files stored by other apps or from making changes to the device. Sandboxing is designed to prevent apps from gathering or modifying information stored by other apps.

Security of runtime process in iOS and iPadOS - Apple Support

The sandbox on an iPhone is a security feature that creates a restricted environment for each app to run in isolation from other apps and the operating system. It is a core component of iOS's security architecture and plays a crucial role in making iPhones more secure.

If you doubt the authenticity of the information provided earlier, you have two alternatives:

1. Report the hacking incident to local law enforcement authorities and actively pursue the case.
2. Accept the credibility of the information; it is impervious to hacking. Just as some individuals hold unconventional beliefs, such as a flat Earth or moon landing denial, one has the freedom to believe in anything. The choice ultimately rests with you in this open and free world.

With all due respect @SravenKrA, iPhone's can and have been hacked using Pegasus malware.

You also did not mention how LockDown Mode limits the attack surface.

The fact that Apple advises users to update their iOS software when it has been released is further evidence that there has been weaknesses in the OS allowing potential penetration with the relative attack vectors being exposed for an unknown duration and a security patch only being available when it gets released.

Justin Perth wrote:

With all due respect @SravenKrA, iPhone's can and have been hacked using Pegasus malware.

Such exploits are typically fixed when known and are incredibly expensive (hundreds of thousands to millions of dollars). Very few people are worth the effort and that is not the case here.

You also did not mention how LockDown Mode limits the attack surface.

Lockdown mode is very limiting. The average user does not need it.

The fact that Apple advises users to update their iOS software when it has been released is further evidence that there has been weaknesses in the OS allowing potential penetration with the relative attack vectors being exposed for an unknown duration and a security patch only being available when it gets released.

That's why there are now, and always have been, security releases.