making cylinders from cubes?

I’m making a space ship and so far I’ve come a long way by making extrusions and grabbing vertices’s, sides, and faces. I want to make the smaller details such as pipes and hatches but I don’t know how to begin. For the pipes, how can I make one without having to create a new object (cylinder) and manually connecting each vertex.
If I’m making these extra details should I be trying to build them from the space ship mesh or should I make new objects? Thanks, God bless and God speed.

Normally I make those secondary details out of new object. That way you can use it all over by making it a group. Would I make it from parent mesh or cube? It depends on geometry of parent object and how detail needs to “fit.” If a fit is on to a complex surface its often quicker to start off from parent mesh.

You can also use “add cylinder” while in editmode and the cylinder will belong to the same object.