When scanning for Bluetooth devices from an app or gateway you will usually pick up lots of iOS devices.
Every Bluetooth LE device, including iOS, has a unique MAC address. MAC address randomization is used on iOS so it’s not possible to track a particular device over time. However, there have been studies that have shown other Bluetooth information can be used to fingerprint devices.
The Bluetooth advertising uses a proprietary protocol and has no use for anyone other than Apple. The advertising is used to provide for what are called continuity messages that allow handoff of tasks, such as writing email, universal clipboard, making calls from a another Apple device, instant hotspot, auto unlock from Apple watch and photo transfer between Apple devices.
An iPhone only advertises if it is associated with an iCloud account to which at least two devices are registered. Advertising can be manually turned off in the Settings Menu. Disabling Bluetooth from the Control Center does not stop the transmission of continuity messages.
It’s surprising iOS devices advertise so often, even when continuity messages aren’t being used, because it uses a lot of battery power. This must be the cost of being able to provide the app handoff messages without the user having to initiate a manual action at both ends. Maybe Apple will be able to overlay the new ‘Find My’ messages in same or similar Bluetooth advertising so as to make best use of the battery.