DeclaraCAD is built on OpenCASCADE. At a very minimum you should understand the basic topology used by OpenCASCADE as it's often needed to build shapes which depend on parts of another shape.

These are:

Shape	Open CASCADE Technology Class	Description
Vertex	TopoDS_Vertex	Zero dimensional shape corresponding to a point in geometry.
Edge	TopoDS_Edge	One-dimensional shape corresponding to a curve and bounded by a vertex at each extremity.
Wire	TopoDS_Wire	Sequence of edges connected by vertices.
Face	TopoDS_Face	Part of a surface bounded by a closed wire(s).
Shell	TopoDS_Shell	Set of faces connected by edges.
Solid	TopoDS_Solid	Part of 3D space bounded by Shells.
CompSolid	TopoDS_CompSolid	Set of solids connected by their faces.
Compound	TopoDS_Compound	Set of any other shapes described above.

I recommend scanning through the bottle tutorial to get a better idea of what these mean.

The actual OpenCASCADE shape created can be retrieved from any DeclaraCAD declaration using the proxy.shape attribute. Most of the time this is not needed but sometimes it is.

The topology of the can be explored using self.topology.<type>. For example self.topology.faces will retrieve a list of all faces within the created shape.

More documentation can be found at OpenCASCADE Documentation.