When I turn on Airplane mode on my iPhone, it switches bluetooth into that "not quite off" mode that you get if you just tap the Bluetooth button in the control center (the icon goes white rather than a cross through it like if you turn it off completely in Settings). The Not Quite Off mode IIRC allows stuff like Apple Watch, Apple Pencil, Handoff, Find My, etc[0] to use Bluetooth but it disconnects all the paired Bluetooth devices you are using (basically it solves that "damn my phone is still connected to the headphones in the other room!" problem that 90% of people turning Bluetooth off are trying to fix)
[0] Full list https://support.apple.com/en-au/102412