Preventing unauthorized access to iPhone screen

Access requires your approval, so without your approval they are not using your phone. Any online account such as Facebook, Instagram, or other Social Media can be viewed on another device with just your Password for that service. Change that Password and they will no longer have access. Be aware of phishing emails/messages where people attempt to get this information.

Recognize and avoid social engineering schemes including phishing messages, phony support calls, and other scams - Apple Support

You can use Safety Check to review other devices signed into your Apple Account and also information that you have approved to be shared with others. It also allows you to change your Password and your Passcode to make sure the device and account is secure.

Safety Check for an iPhone with iOS 16 or later - Apple Support

If you see something on your device that you do not understand, provide a screenshot as long as it does not show your personal information.

Use passwords and passcodes only you know. Don’t re-use passwords and passcodes.

Here are some tips for reviewing your privacy and security settings: Personal Safety User Guide - Apple Support

Usual way iPhone gets breached is through its owner. Through us. We ourselves get hacked. Not our iPhone. We give out our password, we pick an obvious passcode, we get phished or socially engineered, we read some advertisement or some scam and get convinced to do something we shouldn’t. We ourselves are way easier to hack than are our iPhones.

Here is how to: Recognize and avoid social engineering schemes including phishing messages, phony support calls, and other scams - Apple Support

Yeah, and for completeness, iPhone can be hacked, but that’s been very rare and very targeted, and that involves exceedingly expensive tools, based on available reporting.

Put a password on your phone.

Use a passcode with your iPhone, iPad, or iPod touch - Apple Support