Others have mentioned a few specific distros. There are also tools like https://github.com/RedBearAK/toshy or https://github.com/rbreaves/kinto which make the keyboard and other aspects of the OS behave more Mac-like.

For me, Toshy's out of the box config make it pretty painless to switch between MacOS and Asahi on the same machine.

I believe there are a few projects like that:

https://elementary.io/

https://pearos.xyz/

https://ubuntubudgie.org/

Probably better to select and contribute to one rather than starting your own

Unfortunately those are all so far from replicating a macOS experience that the work needed to fill the gaps is about as much as would be needed starting from scratch. Resemblances in all three are surface level at best, and are far from complete even in that aspect.

Such a project is something I’ve daydreamed about on many occasions, but the scope is quite daunting, especially if one wants to do it right and e.g. make sure that all core utilities adhere to the HIG and actually populate the global menubar for example.

What about “Pop! OS” ?

Don’t forget the awkward underscore in the name: “Pop!_OS”?