if you are worried about any of the sensors on your phone always being on, remember that Apple and Google need their relative OSes to make battery charges last as long as possible. So always on stuff is problematic. For a good user experience Bluetooth and wifi usually need to be on, but GPS, camera and microphone usually don't.
For iOS the GPS chip itself is only turned on when 1) a foreground application is using location 2) a navigation app is using location and you task switch (that blue bar you get at the top) 3) for background location, when the device reassociates to a new cell tower
This isn't true, an app has to explicitly turn GPS hardware on. Almost all the time apps, foreground or background, get locations passively, which is done by triangulating cell tower positions, i.e. the location the phone already knows.
How is always-on useful for Bluetooth? In fact, it's a huge vulnerability if that's the case. On Android I simply turn it on to connect to something and off to disconnect.
You can do that on iOS too -- and discoverability is, as usual, a transient state that you invoke manually. So most people probably leave BT on, but not discoverable, most of the time.
