Where to find it:

NodeGraph / Right Mouse Click / Add Nodes / Math / Extension Pack / D /

dot returns the dot product of two vectors, x and y. i.e.,  x[0]⋅y[0]+x[1]⋅y[1]+..

Component Selection can be performed in the Node Properties

https://www.khronos.org/registry/OpenGL-Refpages/gl4/html/dot.xhtml

•X

Specifies the first of two vectors (RGB)

•Y

Specifies the second of two vectors (RGB)

•Output

Returns the dot product of two vectors