How can I switch applications on iPhone or iPad by keyboard like "Windows key + tab" on Windows?
For iPad, ensure that multitasking gestures are turned on in "Settings > General > Multitasking Gestures". Instructions to switch between applications are right below the button.

IPhones do not support task switching with a Bluetooth keyboard.

On iPhones equipped with a 3D Touch screen, you can switch applications by force pressing on the left-most corner of the screen. This will open the task switching view.