In the new iOS 11 Control Center, you can easily turn on/off Wi-Fi and Bluetooth. But actually, when you switch off the Wi-Fi and Bluetooth, these features just disconnect from the current Wi-Fi network/Bluetooth pairing but still running in the background.
Apple makes them always on to keep certain features intact, like AirDrop, location services, the connection with Apple Watch etc. So with iOS 11, your device will try to reconnect to known devices using Wi-Fi/Bluetooth. It will also lead to battery draining and security risk. If you dislike this change, go to the next part to see how to turn off WiFi on iOS 11.
How to Fix iOS WiFi Always On/Turns On by Itself
iOS 11 WiFi turns on by itself is annoying, then how to turn off WiFi completely? You need to go to Settings app to make it
Step 1. Open Settings app on your device > Choose WLAN.