This answer is a bit confusing. In front of the purple line, it reverses S and B. . . Back to your question, the first purple line is the definition of the angle between a straight line and a plane. The second purple line is a regular tetrahedron: if a vertex is taken as the vertical line of the bottom surface, the intersection of the vertical line and the bottom surface is exactly the vertical center of the bottom surface. In other words, the vertical center of the bottom surface is exactly the projection of the vertex on the bottom surface.