Q: how do i  move my data from my old iMac to my new one?

Move your content to a new Mac - Apple Support

I think, when the machine first starts up, you will/should see Start Assistant. I think the difference between the two is

Migration creates a different user and Setup allows you to keep the same user.

It is the same. As long as you enter the same username as previous, the data will be copied over directly for that user.

thanks good to know since I'm expecting a new one today, hopefully.