May I ask if ruuvitag has a fixed device id, and the id obtained from different mobile phones is the same? Thank you for any help
MAC address is same for the same device. However, iOS devices obscure the Mac address. Data format 5 has (known as RAWv2) has the Mac address in the end of the payload for iOS devices, you can use that data to identify tags over Bluetooth. You can get FW version 2.4.2 at lab.ruuvi.com/dfu to try out the RAWv2 format, and specification of data formats is in https://github.com/ruuvi/ruuvi-sensor-protocols .
You can also read the Device ID (which is different from MAC) with NFC.
About the UUID of ruuvitag