How do I protect my iPhone from getting compromised?

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.

1. Immediately change your Apple ID password
   1. Go to Settings → [your name] → Sign-In & Security → Change Password.
   2. This signs out anyone using your account.
2. Enable Two-Factor Authentication (if not already): Same menu → Two-Factor Authentication → Turn On.
3. Check for any unknown devices linked to your account:
   1. Settings → [your name] → Devices.:
   2. If you see anything you don’t recognize, tap it → Remove from Account.
4. Check for unknown profiles or device management:
   1. Settings → General → VPN & Device Management.
   2. If you see a profile you didn’t install, tap it → Remove Profile.
5. Check Accessibility & Screen Sharing settings
   1. Settings → Accessibility → Switch Control, Voice Control, or Remote Control — turn off anything you didn’t set up.
   2. Settings → General → AirPlay & Handoff → make sure AirPlay is set to Ask or Never Automatically.
6. Sign out of iCloud everywhere and reset
   1. Go to Settings → [your name] → Sign Out (you’ll need your password).
   2. Then back up your photos to iCloud or your computer.
   3. Go to Settings → General → Transfer or Reset iPhone → Erase All Content and Settings.:
   4. Set it up as a New iPhone (not from a backup), so nothing suspicious is restored.
7. Update iOS: Always keep the latest iOS version for security fixes.

If a person has ever had your passcode, they could’ve installed something quietly — so the full erase and setting up as new is the safest option. How to factory reset your iPhone, iPad, or iPod touch

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

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

No, your iPhone was not hacked. Perhaps your Apple Account credentials were compromised. See https://support.apple.com/en-us/102560

Getting nasty messages from neighbours or enemies is not being hacked.

In addition to the other advice

Change your Apple email address https://support.apple.com/en-gb/109353

Use that address (icloud email) for banks emails etc and nobody else

Get a new phone number.

Don't use social media, find out how to close social accounts.

Control access to hardware features on iPhone:

https://support.apple.com/guide/iphone/control-access-to-hardware-features-iph168c4bbd5/ios

Your phones were not hacked. The police are correct.

You're being scammed.

Re: “The hacker warned me to move because he was going to make me sorry. And that he had pictures of me.”

Sounds like a common scam, probably a sextortion scam. Criminals send you (and many others) messages that are packs of lies - messages saying that your devices had been hacked and that the “hacker” )scammer) has a hold over your device and you. Messages saying they are going to expose you to the world if you don’t pay.

They have nothing, but are hoping to panic their victims into acting without thinking, and paying the blackmail. Since it is cheap for criminals to spam thousands or millions of people, they only have to fool a few victims to make the crimes profitable for them.