How do I get file such as texture out of already packed blender file? So I downloaded some .blend files, and it came in .blend files with packed texture file (meaning the texture is not in separate files or folder, but packed inside the .blend file).

How do I get the texture from this 'already packed .blend file' out of it in a separate file so I can apply it to my other 3d model ?
You can either go in the UV/Image Editor, and, in the bottom horizontal menu, select the image you want to save un the drop-down menu, then Image menu > Save As Image... or in the Node Editor, click on the Open (now Unpack) button of the Image Texture node: as the images have been packed, this button will unpack the image the way you want