How to construct transition or spool piece

Since you provided an assembly model, my solution assumes you want to make a 3rd component to connect the round flange on one part to the rectangle flange on the other part. If that is the wrong assumption, and you wanted a single part that includes the round and square flanges you provided,that's ok, because the technique would be pretty much the same.

Step 1: save that assembly as a part file and work in 'multi-body part' mode. The model will be less finicky this way.

Step 2: create the mating flanges. I left the holes out to be added later. Don't forget to un-check 'merge result' in order to keep the mating flanges as separate bodies from the provided flanges.

Step 3: create separate sketches of the inner profiles and 'loft' them together (un-check the always defaulted to 'merge result')

Step 4: shell 'outward' the desired thickness of the transition

Step 5: 'combine' the transition body with the mating flange bodies, note the 4 undesired scrap surfaces resulting from the combine

Step 6: 'delete face' on the scrap surfaces to eliminate them. I used 4 features but this could all be done with a single 'delete face' feature.

Final steps: I didn't do these steps, but the bolt holes still need added to the mating flange, maybe you want some fillets between the transition and the flanges, and lastly the original provided flanges get deleted with a 'delete body' feature, leaving you with a single part that transitions between the provided flanges.

That's all there is to it! There are other methods, but this seemed to be the most straight forward to me.

Good luck and have a nice day! :-)

If you want to use the sheet metal for manufacturing, it will be necessary to cut the sketches before sweeping the tolery! See the example of my file.

The transition between the square and a circle is easy with the sheet metal SW but it is necessary to know that the result will be a rectangle with fillet corners and the circle will become a polygon with the necessary number of folds.