Regarding runner and pipeline

[Sashikanta Mohanty]

I have a question regarding Add Runner/Runner installation. When I Add a runner for Example: select Windows(64bit) for System and Architecture, I see Runner labels comes "self.hosted" and "windows". 
Question1: It is not allowed to delete these default labels. Is it actual behaviour or any way to delete any one runner from these two?

When I run the pipeline and passing 
runs-on:
- self.hosted
- windows
It is working.
Question2: When I keep both Its working and when I delete "windows" and kept "self.hosted", It gives error "no runner is active" even my window runner is on. But when I have put only "windows" as runner It works. Is it the behaviour?

Question 3:
How operating system is getting detect. If I have 2 steps and in one step I am taking 
runs-on:
- self.hosted
- windows
means with runs-on and trying get OS type by using windows command, It is giving me proper result. and in second step I am not passing any runner. when I am trying to execute 
- OS_TYPE=$(uname -s)
- echo "Detected OS- $OS_TYPE"
It gives me "Linux".
Not able to understand in second step how It is getting operating system?

Question 4-  
I have a scenario when I am not passing anything through runs-on then we are considering it as cloud runner. So, In this case   how a system will understand which command to execute If Its window/linux/mac. Please suggest. As per my understanding It will run only linux command . So in case of windows cloud - how can I manage?

Please check those scenarios and give me some solution. I am getting stuck because of this clarification.