Maybe this is because of my long time use of Windows but my device usage often requires me to switch back and forth between 2 apps or windows.
Neither Android or iOS have readily supported this. Recent Android releases require you to tap the task list key and then tap the app you want to switch to. Nougat improves this letting you double tap the task list key to simply switch to the previous app. iOS has required you to double tap the home key and then swipe back to the previous app and tap on it.
Finally 3D Touch addresses this in iOS for the devices that support 3D Touch.
To do this just press firmly on the left side of the screen. Then swipe all the way to the right to go back to the previous app.