> haven't found much that explains how or why it's different

Windows are directories in the filesystem. Their properties are files inside it. Modify the file, the window changes.

This means that to open a window on another machine is trivial. Machine A mounts the relevant part of the filesystem on Machine B over the kernel's integrated 9p networking stack, then creates and populates a folder, and now the window is on the other machine.

No "servers" or "clients", no special network protocols, works locally or across the planet, according to appropriate permissions of course. Definitely no need for Windows-like crude hacks such as VNC or RDP.