Ultimately X11 is the main component that you use to draw and interact with a GUI on a traditional Unix or on a Linux system (Wayland is a more recent alternative, and there have been other attempts in the past, but X11 is still the most commonly used).
There is no 1:1 correspondence on Windows. Some of what X11 does is part of Win32, other parts are in explorer.exe, others are built in at deeper layers; there are multiple alternative systems available on Windows too.
Ultimately what X11 gives you as an app is a way to draw something on the screen, and to get input events from users. X11 also coordinates this between multiple separate apps, so that you get movable windows and focus and other such behaviors without having to have each app coordinate manually with every other GUI app. Copy-paste is another similar cross-app functionality that X11 offers.
The way X11 does this is through a well defined protocol. Instead of relying on system calls, you open a socket to some known port and send drawing commands there, and receive input events from it (of course, many libraries abstract this for you). Because of this, it can work transparently regardless of whether you are drawing on the local machine or on a remote one. So, X11 itself can work as a Remote Desktop solution as well without the need of a separate program or protocol (though there are significant differences with pros and cons).
There is no 1:1 correspondence on Windows. Some of what X11 does is part of Win32, other parts are in explorer.exe, others are built in at deeper layers; there are multiple alternative systems available on Windows too.
Ultimately what X11 gives you as an app is a way to draw something on the screen, and to get input events from users. X11 also coordinates this between multiple separate apps, so that you get movable windows and focus and other such behaviors without having to have each app coordinate manually with every other GUI app. Copy-paste is another similar cross-app functionality that X11 offers.
The way X11 does this is through a well defined protocol. Instead of relying on system calls, you open a socket to some known port and send drawing commands there, and receive input events from it (of course, many libraries abstract this for you). Because of this, it can work transparently regardless of whether you are drawing on the local machine or on a remote one. So, X11 itself can work as a Remote Desktop solution as well without the need of a separate program or protocol (though there are significant differences with pros and cons).