Is there any other way to make a 2D city without the need to model 2D planes in a shape of buildings?
Well, you can make a 3D city, render it with transparent film and use it as 2D city… 
This depends. If you have a still image you can obviously use a single image with a city on it.
If you have an animaton, you can get away with a single image if you have little camera movement and if this camera movement is horizontally. As soon as you have vertical pans and tilts a single image will usually not be enough.
If it’s just for a distant background, take a photograph of a city skyline, open it in Photoshop, select the sky/background and delete (so there is nothing there, no background layer etc.) Save with alpha/transparency and use as a texture in Blender with transparency. ‘Import Images as Planes’ addon in Blender may help here.
The harder part will be getting it to match the lighting of your scene.
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haha heey maan now that’s what I call a solution