How to detect if someone is tracking my iPhone?

• Check if someone else knows your Apple Account password and is signed into your account by going to Settings > [name on top], and scroll to the bottom to see devices signed into your account. You can remove a device by tapping on it.
• Go to Find My > Friends, and if you previously allowed a person to track your location, when you tap on them you will have the option to Stop Sharing your Location.
• If someone planted an AirTag on you or in your car, you will automatically get a notification that you are being tracked.

No one is tracking your phone, unless you gave them permission to know your location in Find My - Friends. If you haven't given anyone permission there, no one is tracking you.

While it is highly unlikely that any individual is tracking your phone unless they know your Apple Account credentials, your carrier is tracking you. They are tracking your location, the cell towers you use, the numbers you call and message, how much data you use and, in certain cases, what websites you visit. There is nothing you can do about this.