what does bluetooth transmit when it is just visible

For classic Bluetooth (i.e. pre Android API level 21), you are restricted with the data that you can send in the inquiry response (which is connection-less data sent over the air). There are three types of inquiry response data that can be sent over the air:-

Inquiry Result:-

Inquiry Result with RSSI:-

Extended Inquiry Result:-

In other words, depending on what the scanning device asks for, the result can be different, and even though in all cases it is not just name and mac address, you cannot add just random extra data to the inquiry response.

For Bluetooth Low Energy (i.e. Android API level 21 and later), advertising data can be dynamic and can be configurable. The minimum advert report can only contain the device's Bluetooth Address (either the public or the random address). In other words, even the name of the device is not a requirement. However, it is usually common to include both the address and the name as this gives more information about the advertising device.

You can read more about this in the Bluetooth Specification v5.2, Vol 6, Part B, Section 2.3.1 (Advertising PDUs). Also have a look at the links below for more information:-

• Is a BLE advertisement required to provide a non-empty local name
• How BLE works: advertisements
• BLE advertising primer