Creating a 3D Model with Tinkercad - 3D Printing: Build your Own 3D Printer and Print your own 3D Objects - page 135

Next, I use the Align tool again to force the Round Roof to share the front edge of
the rectangle. Figure 7.10 shows that I'm clicking on the lower Align dot so that
the Round Roof will move to share that edge.
Figure 7.10 Align the two objects along a common edge.
Figure 7.11 shows the result. You'll also see a copy of the Round Roof object. I
selected just the Round Roof and made a copy of it using Control+C and pasted a
new version with Control+V.