Cross Vector

source：https://www.cuemath.com/geometry/cross-product/

Unit normal vector of a surface

source：https://www.khanacademy.org/math/multivariable-calculus/integrating-multivariable-functions/flux-in-3d-articles/a/unit-normal-vector-of-a-surface

Normal Vector

source：https://mathworld.wolfram.com/NormalVector.html