How do I stop someone who illegally y gain access to my iPhone

No one has access to your phone unless you gave them the phone for few hours and also provided them with your Apple ID and Password.

If you allowed another person to "share" your location information, then they can track the location of your phone from their phone or computer.

Are you sure that you are not seeing a scam message or email that is trying to trick you into buying useless software that will do nothing to help your phone?

Use the built-in privacy and security protections of iPhone - Apple Support

Wolfplayer2114 wrote:

or for example have you plugged into a public charging station in an unsafe environment?

I know your intentions are good but please don't contribute to the media scaremongering about using public charging stations.

https://slate.com/technology/2023/04/free-public-phone-chargers-fbi-warning-bad-actors-threat-bogus-debunked.html

Why do you believe this is happening?

What are you seeing that makes you think this is true?

Has your phone been stolen or anything like that or for example have you plugged into a public charging station in an unsafe environment?