Google appears to be building out a feature in Android 15 that automatically turns Bluetooth back on every day if you turn it off. Strings in Android 15 builds spotted by the folks over at Android ...
Turning off Bluetooth and Wi-Fi does reduce battery usage, but not enough to worry about it.
If you turn off Bluetooth on an iPhone through the Control Center interface, it doesn’t actually turn Bluetooth off. The Bluetooth and Wi-Fi controls in CC only disconnect devices from those services.