How can I disable Edge open at startup?

First place to look IMHO is in System Preferences >> Users & Groups >> you account by name. On Right a Tab marked Login Items and inside look and see of Edge is inside. If Yes, small minus ( -) and hi-lite Edge and click the minus sign will remove from list.

To Test - restart computer and see if Edge starts at bootup

You will want to open it in finder and go to preferences and navigate to startup items or if you use Mac keeper it will take you to it. And Mac security section will also be a place to turn it off as well

Agee with @ Old Toad regarding " MacKeeper " - and please Remove as per Developers Specific Instructions.

Any Third Party Applications that will interfere with the normal operation of the OS, alter, modify, remove or delete or attempt to do so is an invitation for disaster and may require a Reinstallation of the OS.

And MacKeeper falls into the above Category

I’m not able to edit the post but I was told to remove the fact to use Mac keeper. It is a third party app and should is an invitation for disaster. Please disregard what I said about Mac keeper and carry on with your hopefully amazing day. :)

I’m trying to edit the post to remove the Mac keeper but there is no longer an edit option. I wrote a comment saying to disregard anything I said about Mac keeper and I’m so sorry! I made a mistake about mentioning Mac keeper. I will not mention 3rd party things like that :( sorry

Don't use MacKeeper as it's considered to be akin to malware by most of the experienced contributors to these forums. Follow P. Phillips' suggestions.

Thanks ;-))

Welcome and Thank you too :-))