Create faces, to get 3D intersection spline

[SCALEMODEL]

Good tutorial here on 3d tube/pipe

Drawing/sketching spline between planes (flexible tubing)

Here’s an example. I selected both beginning and ending circle edges. Note at the bottom lower right of the screen there are measurements- deltaX, Y, and Z (shown just before the transition). It gets a little more complicated when the two pipes are at different angles to each other but still doable.