Left panels: age-metallicity distribution of the masked solutions for ks_thick disc stars (top, represented by individual red points) and ks_thin disc stars (bottom, shown as a blue-scale hexbin density diagram). This is the same solution as Figure 4 bottom panels, i.e., the distribution of synthetic stars from the best fit which corresponds to the age-metallicity distributions of the living stars in the volume analysed. Right panels: [Mg/Fe] versus [Z/H] (global metallicity; see Section 3.2 in the main text) based on spectroscopic measurements from APOGEE DR17 and GALAH DR3, homogenised using the SpectroTranslator. The ks_thick disc stars are displayed at the top (with individual red points), while ks_thin disc stars appear at the bottom (as a blue-scale hexbin density diagram). The isocontours in red (blue) display the 30%, 60%, and 90% percent of the stellar distribution in the chemical space corresponding to the ks_thin disc(ks_thick disc) in addition to the ks_thick disc(ks_thin disc) stars. The vertical black dashed lines indicate the metallicity ranges of interest discussed in the main text.