The site you linked does not track your phone via its IMEI. If you read the instructions carefully, it has nothing to do with the IMEI.
The IMEI Tracker by [vendor] is an online tool that helps you to track lost/stolen Android or iPhone using IMEI number in real-time on Google Map.
Open the IMEI Tracker by [vendor] tool on your phone. Type 15 digits number in the given box and tap on the track button. Next, just select a live location option, and you get to see your phone’s location on Google Maps for free.
You install an app on your phone and set it to "live location mode". That app sends GPS coordinates to the vendor's server, and you can look up the current location by IMEI on the website. You're purposefully providing the vendor with your phone's location, rather than them somehow tracking it via the cellular network.
Precise location tracking as a cellular network feature is only implemented via Advanced Mobile Location (AML), a technology built into modern phones that is turned on when you make a call to an emergency services number. The phone uses GNSS (more commonly referred to as GPS) to find its position, and relays that information to the emergency services to aid with locating the caller. AML is mandatory for devices sold in most regions.
More general location tracking can be performed by the cellular network by looking at which tower your phone is currently registered with, but that only provides a rough area and it isn't always useful. While cellular location data has become a popular plot point in crime shows, leaving people with an expectation that it is an accurate way to say whether a person was in a particular area at a particular time, its reliability as a location indicator is fairly low in practice. Real-time cell tower registration data is not supposed to be accessible to anyone except the provider and network operators. In reality, access to that information has been known to be illicitly sold, but the practice is generally illegal.