Posteriors of the internal structure parameters obtained for TOI-421 b, namely the inner iron core, silicate mantle, and water layer mass fractions with respect to the condensed part of the planet without the H-He envelope, the molar fractions of Fe, Si, and Mg in the mantle and Fe in the inner core, and the logarithm of the mass of the H-He layer in Earth masses. Each column is labelled with the median of the corresponding posterior, with the 5^th and 95^th percentiles as uncertainties.