The Messages app opens whenever I boot up my iMac

Greetings Serqet,

If applications are starting when you power on your Mac, you likely have login items in place. You can learn how to remove those login items here:

Remove login items to resolve startup problems on your Mac

You'll just need to focus on removing anything you do not want to open when you sign in to your user account on your Mac.

On your Mac, choose Apple menu  > System Preferences, then click Users & Groups .

Click your account name below Current User, then click Login Items at the top of the window.

Make a list of the login items—you’ll need to remember them later.

Select all of the login items, then click the Remove button .

Choose Apple menu > Restart.

Take care.

I have the same thing: I get a pop up whenever I boot up. In my case, I believe I figured out the cause (and no, I have absolutely nothing listed in login items): I have never signed into Messages; in fact, I've never even opened the app. I believe Apple - in its quest to use your Apple ID for all things Apple whether you want them or not - is trying to force me to set up/sign in to the app. That I will not do, so I just dismiss the prompt.

Right Click on the Messages Icon in the Dock and make sure that Open at Login is UNCHECKED

Signing into icloud.com

If you have to do this when logging into icloud.com on a browser it is probably because you haven't agreed to" Trust This Computer/Browser"