[John Scheffel]

You might want to look in Help/On Topics and search for "catch" to learn about Catch modes (CoCreate term for snap mode).

You can catch the center or circular elements or straight edges by changing the catch mode to center or using Ctrl-Shift-LeftMouseButton.

You should be able to catch 2D elements in work planes when creating 3D lines, so if you create workplanes at ends of the tubes you should be able to catch points in them. You might want to check out the Contour menu and the Project options. These can be used to project 3D elements into a workplane. With these you should be able to create geometry in the workplanes to define the center points of your tubes.