Apps Startup on Power Up When I turn my phone on, many apps also turn on in the background. This slows down my phone. How do I shut them down? Why is the OS built this way? Doesn't make sense to me.
The apps which start automatically with your phone have it in their permissions to start together. Apps like Messengers need to start with your phone so as to be able to run in the background and show latest messages received etc.

You can try an app called Greenify(which can hibernate apps in the background or permanently). Give this a read.