Whenever your phone device is in range of a known WLAN, the Wi-Fi connection will take priority over mobile data except for some scenarios and applications which specifically need to use mobile data, like when you are sending a multimedia message.

Once a connection is established, the phone will not change the connection type during use. If for example you have an application actively connected via mobile data you will have to close the application and restart it to get it to reconnect over Wi-Fi.

If your Wi-Fi network connection is lost, the phone automatically tries to connect to a different Wi-Fi network or use mobile data if this is turned on and no other known Wi-Fi is in range.