The Tube Handle tool allows you to create a cylinder-shaped connection between two selected areas. This tool requires two distinct input selection regions, as shown in the image below. The entire selected areas are discarded, creating two holes. These holes are stitched together with an initial cylinder-shaped mesh, which is then remeshed to create a high-quality connecting tube.
The property pane for Tube Handle is shown to the right. The Refine slider controls the number of iterations of mesh refinement (see the Remesh tool for details).
The Flip Orientation checkbox can resolve an issue where in some cases the loop correspondence is incorrect, and as a result the tube is "pinched". If your results are unexpected, try checking this box.
The example below shows quite a simple case, where the selection regions are nearly discs. However, Tube Handle can deal with cases that are much more complex. Nearly any pair of boundaries can be stitched together. However if the boundaries are very different in shape, then it is likely that the connecting tube will be unpredictable.