Well, normally you would just produce 1 graphic at the highest resolution you will be publishing to. Then you would use the Unity tools for setting mip-map levels. You would then modify settings like 'pixel perfect' or 'texture type' (compressed, truecolor, etc.) to get the best look for the compression you choose.
Creating resolution independent sprites is tough and memory consuming.
What do you mean?
You're either creating your graphics in a vector program like Inkscape or Illustrator, or you create them in a raster program like Gimp or Photoshop. The difficulty is only determined by your comfort level with your toolset.
How can .svg assets be used in unity for any kind of 2d game?
With a plugin: https://www.assetstore.unity3d.com/en/#!/content/38258
What are alternatives to .svg that can be used in unity?
You can use the Unity LineRenderer or standard GL drawing methods. This Post has some good examples.
Can sprites with various shapes be generated within unity?
They will be whatever shape you draw them.
Would like creating universal art asset that'll support all devices with different resolutions.(we'll handle the aspect ratio )
This isn't a sentence. But, Unity won't change aspect ratio, unless you scale on only one axis.