why the mesh moves from its position with curve modifier When i add a curve modifier stairs moves from its position. How it works can u explain in detail.How to set correct position from where i have the exact position i have set. Rotation scale and position are all 0,0,1 of stairs and curve. In some cases why we have to move and adjust. Any suggestion or help. Thanks
You can solve this problem by following the steps:

Set the origin of the curve and the stairs to the geometry
Move the stairs downward on the x-axis to where the arrow is
Rotate it on the y-axis to be parallel to the arrow (Do it in the front orthographic view
move it along the x and the z axis repeatedly to find the best fit.
Which will give you this:

For getting a much more refined curve I added in another nurbs curve and did a lot of tweaking. Added in plane.Modeled it into a stair added in an array modifier and a curve modifier.And finally this is what I got:
Here is the .blend file
https://drive.google.com/file/d/1nvo0X5z172oPs1BdGVXNDQF2nUX20u_j/view?usp=sharing