DHH switched from Mac to Linux and is in the process of experimenting with his setup, but since he's famous within tech, it's getting a lot of attention. There's really nothing special about it.
In this case, Walker is the launcher being used in Omarchy, https://github.com/abenz1267/walker. It's not specific to Omarchy.
One of Walker's features is being able to create your own custom menus quite easily with shell scripts.