What exactly goes into PRs you've seen? How do they codify these steps, such that: - Release steps can be tested while PR is being developed. That is executed

What exactly goes into PRs you've seen? How do they codify these steps, such that: - Release steps can be tested while PR is being developed. That is executed somewhere, which doesn't interfere with production or ideally other developers ongoing PR should it go bad - Upon merge production system safely transitioned from one state to another without human intervention. Steps, for simplicity are: - Deploy new version of the code behind new load balancer - Make some requests through it to verify its workig - Switch DNS to the new load balancer IP - Wait for old load balancer new connections to die down and remove both LB and old version of the code behind it.