We know how to blend these two cross sections using loft option. Let us see some advanced features of loft using which we can give a path that these profiles that is suppose the circle or the rectangle are going to follow and blend with each other. For this I'm going to take a plane which is perpendicular to these two planes and draw a profile which is going to pass through these two profiles. So my spline is going to join the store profiles. Here I'm taking another random spline to make this product a little bit more complicated. Let me take another plane that is perpendicular to these two planes.

And here again, I'm going to use the project option. Get the points on the profiles, and use them as starting and ending points for drawing rails. So now I'm going to stop sketch, I'll be switching off these planes so that is clearly visible. So here are my cross sections and the path. So I'm going to create loft. So these two are my profiles.

If you see the option rails, I'm going to select in the dialog box rails and going to select this Planes This is another rail. So you can see it is going to pass through that plane. So rail is basically going to control how these two cross sections are going to blend with each other