The movie demonstrates several types of transformations: a homothety, a symmetry transformation relative to a line, a rotation.
A point light source (lantern) makes a shadow from the triangle on the wall. This transformation is called a homothety. In this case the point light source is the center of the homothety.
Then a triangle reflects from a mirror, the triangle has symmetry transformation relative to a line (the table surface).
The center of the rotation presented in the last example is the stick endpoint.