My Samsung turns off bluetooth if nothing is connected to it but doesn't if something is. Pretty hand for when you're listening to music on your flight and you turn on airplane mode.

The precise behavior of Airplane mode on iOS / WatchOS is dependent on the user’s interactions. The default is to turn off everything but Bluetooth. But, if you turn Airplane mode ON and then turn Wi-Fi on, iOS will remember this preference.

You can learn more about Airplane mode on Apple’s website:

https://support.apple.com/en-us/HT204234

It doesn't completely turn off Wi-Fi either, it just disconnects from the current network.