When I try to turn iPhone 11 off , the siri app appears, not the Apple symbol . So I cannot turn the phone off
You need to hold (either the volume down button or the volume up button) and the side button AT THE SAME TIME, for the "slide to power off" to appear.
Holding down only the side button results in Siri appearing, as you have observed.
iPhone 11 won’t turn off. I follow the instructions, but Siri app appears, not the Apple symbol
To turn off your phone hold BOTH the Volume Up and Side button at the same time, until Slide to Power Off appears, then do so. And note that no Apple logo will appear; the phone will just turn off.
And if using the buttons does not work, you can also turn off your phone by going to Settings > General > Shut Down.
My iPhone 11 will not turn off
