Headless app env variables, update app & undeploy app clarification

Hello Adobe team

As i prepare for the final leg, could you please help and clarify the following 

a.) is there any setting/variable that can be used to detect the app builder env (dev/staging/prod) ? This is to switch AEP sandbox within the code from dev to prod on publishing of the app.

b.) i will be using the out of box github CI/CID to deploy this app, to update the headless app do i just push the latest version via CI/CD and that will update the prod app? 

c.) What is the procedure/steps to undeploy the app in worst case scenario? could not find any docs on this. Is this done using github CI/CD? 

Thank you!!