Apple - How can I switch applications on iPhone or iPad by keyboard like "Windows key + tab" on Windows?

If you are using a MacOS/iOS BT keyboard with the Option and Command (aka clover leaf or open Apple) keys, then this should help you out…

iPad Navigation Keyboard Commands Basic keyboard navigation shortcuts on iPad are as follows:

Control+Option+H – Home button Control+Option+H+H – Show multitask bar Control+Option+i – Item chooser Escape – Back button Right Arrow – next item Left Arrow – previous item Up + Down Arrows simultaneously – tap selected item Option + Down Arrow – scroll down Option + Up Arrow – scroll up Option + Left or Right Arrow – scroll left or right Control+Option+S – turn VoiceOver speech on or off iPad App Switcher Keyboard Commands Arguably the most useful set of commands are related to app switching:

Command+Shift+Tab – switch to the previous app Command+Tab – switch back to the original app Left+Right Arrow, then Option + Left or Option+Right – navigate through Dock

Source: http://osxdaily.com/2012/10/01/ipad-keyboard-navigation-shortcuts/
More shortcuts: http://www.iphonehacks.com/2013/11/keyboard-shortcuts-ipad-iphone.html

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.