By pressing the the W key in Tinkercad (or selecting the workplane icon), you can define a new workplane on the surface of an object. It's a handy way to add objects flush against the surface of your design (shapes, text, holes).
This is a necessary technique for placing objects on irregular or curving shapes, but it also makes quick work of stacking common shapes. To reset the workplane back to its default location, simply drag a new workplane back to the original location.
The following video by LinkedIn Learning briefly explores the usefulness of the workplane tool.