vkdt mainly uses the D65 colour matrix for this. it is extracted from the metadata shipped with rawspeed or contained in the dng exif tags of the raw file.
more accurate spectral input profiles can be created if you have spectral response curves of the colour filter array. if you don't, vkdt ships a tool to reconstruct plausible curves which will still behave better than the matrix: gamut boundaries will be handled gracefully without creating imaginary stimuli/negative energy, and interpolation of the input transform for white balancing can be performed more accurately.
the processing of all of this is done in the colour module and the documentation for the
tool to create spectral lookup tables is found here.
in short, it creates a spectral lut from a dcp profile or the same data
embedded in a dng file.
tl;dr:
vkdt-mkssf your.dcp
vkdt-mkclut <camera model>.txt
there are a few options for the optimisation process involved here and there is an instructive report printed as html.