The extended catalogues are produced by finding the theoretical CMD star closest to each observed star in each field and adding these data from the theoretical CMD star to each observed star.
Notice that the cool white dwarfs do not get fainter than F814W=28. This is an artifact of the spectral model, not reality, so don't trust the photometry for the faint white dwarfs.
However, you will be inserting flux at the white dwarf positions to test the completeness in the crowded fields, so you will use your own values for the white dwarfs.
The X and Y positions in the catalogue are in ACS pixels (0.05 arcsecond). The NIRCAM short-wavelength scale is 0.031 arcsecond.