The third sentence, the bright moon decorated your window, actually refers to the people who look at the scenery through the window, and if you look back at the people who look at the scenery, this is your window, and you are looking at the people who look at the scenery through the window. This window allows people who look at the scenery to see you. This window plays a role in letting others see you. This window is made for others to see you, so it is also your window. The third sentence comes from this.
So you watch the scenery on the bridge, and the people watching the scenery look at you upstairs through the window. The bright moon decorated this window that belongs to you, and you decorated your dreams.
This poem is too narcissistic, and Your Window is a bit overbearing. It's just that the author loves to look at a person. How do you know that the person who looks at the scenery is looking at you?
The last two sentences are actually false sentences, and the real artistic conception is:
You stand on the bridge and watch the scenery. The scenery watcher is watching you upstairs. I stood on the bridge and watched the scenery.
The above understanding may also be wrong, another understanding is:
You stand on the bridge to see the scenery, I stand upstairs to see you, and the bright moon decorates the window of your heart. You make me miss you so much.