With the iOS 15 update, if your iPhone is stolen and the thief decides to turn it off, you will even be able to track it. The only way for someone to turn off FindMy app tracking is to delete the iPhone owner’s iCloud account. And for that, the thief has to crack your iCloud account, which would be quite a difficult task.
7 Apple iPhones that will support the FindMy app’s tracking feature when the device is turned off
The reason that only newer iPhones are able to support this feature is because Apple uses Ultra wide band (UWB) Technology. This technology is available for iPhone 11 and above. It’s the same technology Apple uses in Airtags to spot lost items. For older iPhones, you will be able to track it even when the device is cut off from the internet. But if it is off, there is no way to follow it.