Confusion with on X Server [duplicate]

I like to connect to my raspberry pi 4, which runs ubuntu, and use some gui programs. I do this via:

ssh username@ip_of_raspberry -X

This works. But I'm confused whats the value of the display variable shall be. Imho it must be the ip of the machine that runs the x server (in my case a windows machine). So I enters the following on the ubuntu side:

export DISPLAY=ip_of_windows:0

After I did this and I start a program with a gui like thunar, the terminal hangs.

My confusion starts when I use putty instead of my ssh client from the terminal. In this case all works like a charme, even using programs like thunar. But when I echo the display variable there it shows me this:

localhost:10.0

So my question is: which ip address shall I assign to my display variable? The one of raspberry pi or the one of my remote machine that shall display the gui?

Any help is appreciated