Android is a software stack but not an OS. as stated by developer.android.com
What is Android? Android is a software stack for mobile devices that
includes an operating system, middleware & key applications. The
Android SDK provides the tools and APIs necessary to begin developing
applications on the Android platform using the Java programming
language.
I don’t understand that software stack is OS works with the Applications, so Windows is an OS but if I install Applications on it, should I call it a software stack?
So what is the difference between Software Stack and OS, Android includes kernel so why not its an OS?
The OS is the core kernel of the system, and the software stack is the software sitting on top of the kernel which enhances and expands the functionality of the system.
Windows tends to blur the line a bit, but Linux makes it a bit more visible.
In Linux, the kernel is the OS. The software stack sitting on top of the kernel can include things like:
So things like a control panel application to edit system settings aren’t part of the operating system. They’re part of the software stack on top of the operating system.
You could have a very tiny operating system which doesn’t have much of a software stack running on top of it. By itself, it wouldn’t provide much usability, but rather a foundation for adding usability.
Android has its Software stack built upon Linux Kernel, which takes care of the power management acts as an interface between other hardware parts of the device. Here’s a link which can help you learn more about Android architecture