The old Cosmic was built on top of Gnome using extensions, but the new Cosmic was written from scratch. It largely mimics the look of old Cosmic, but has introduced a few new things.
There are desktops try do mimic the look of MacOS, but none I've used actually felt like using MacOS. The first time I used MacOS, I was shocked at how many quirky things it does, the way it operates. No Linux desktop prepared me for that.
It's not atomic or cloud native either if you want to be strict with definitions.