can iphones be tracked in findMy with only bluetooth active?

The answer to your question is "yes".

You might be also be surprised to learn that the phone will send out a brief intermittent Bluetooth signal......even if it has been powered off.....(as long as the battery has some juice left).

Not all phones will do this though. You can find out if yours will by tapping Settings > General > Shut Down and looking to see if there is a message below the slider.

Is there a message there that says.......iPhone Findable After Power Off?

If yes, your phone will send out a Bluetooth signal even if it has been powered off. If other iPhones are in close proximity, they will pick up the signal and and send it along with their location information.

If the Find My network was enabled on them, yes. But it's not the same as if it has a data connection. It reports via other iDevices with the find my network enabled and can't be tracked in 'real time'.

An iPhone that has Bluetooth enabled can be tracked the same way that AirTags are; both respond to another iPhone that comes within 30 feet of the lost iPhone using a feature called Find My Network (there’s a switch to enable it). It is a “crowd-sourced” service, similar to Tiles. The iPhone that senses the Bluetooth signal reports the location of the iPhone that sensed the signal to Apple’s Find My server, and the lost phone’s most recently detected location will appear in Find My on another device or https://icloud.com/find.

It does not show the location of the lost phone directly; it is the location of the last iPhone to pass by the lost iPhone. Depending on other factors, this can be from 30 feet up to about 100 feet from the actual location.