To connect duct or pipe

Select (Connect Ducts/Pipes). The Connect Ducts/Pipes tool settings opens.
Set the Join Method to an appropriate method for the intersection:

Elbow

or
Tee

or
Cross
Key-in a value for a gap allowance in the Tolerance option to maintain connectivity for joints that are not precisely located in the model.
Select the first component.
Note: The first selected is modified (broken, stretched) to complete the joint. First selected is trimmed in tee intersections, and broken for cross intersections.
Select the second component. The joint is created with correctly sized fittings and full connectivity is maintained.