Two different things here. In the Wayland/Xorg world, server-side decorations means its drawn by the compositor or display server, neither macOS or Windows do

Two different things here. In the Wayland/Xorg world, server-side decorations means its drawn by the compositor or display server, neither macOS or Windows do this. Windows is a lot more complicated, the decorations are drawn by the client... but that client is part of the userspace dll from the system you link to in your app and gives you a handle to draw on. macOS the only documented method to draw afaik is through Cocoa which can give you a space to draw on, but the linked Cocoa library will still draw your apps window. To the user, I guess SSD achieve the same thing as Windows/macOS do, but with very different methods because unlike macOS and Windows, there's no central toolkit/library.