Centering one circle on another

I am using SketchUp for iPad and I am trying to build a round display table.

I know this is total newbie stuff but what is the easiest way to make sure the two pieces are centered when connected them together?

The easiest is to use inferencing to draw the parts aligned to begin with.
By moving the pencil near the center of the first circle the tool will snap to the midpoint. Or you could use the offset tool to make a second smaller circle from the first.
If you already have the two cylindrical parts, you can use inferencing again to grab one at its midpoint and snap it to the midpoint of the other.

I don’t have the ability to record my Ipad just now, but what I show here in Pro is pretty much the same on the ipad.