Mass-metallicity relationship of star-forming galaxies. The prediction by Model B shown as a red solid star is compared with observations of a large sample of galaxies. Solid circles refer to the stellar metallicity derived from spectroscopy of individual blue supergiants (cyan), red supergiants (pink), and super star clusters (yellow) (see Kudritzki et al. 2024, and references therein). The open orange diamonds represent the result of integrated galaxy spectra of young stellar population for 250 000 SDSS star-forming galaxies by using a stellar population synthesis technique (Sextl et al. 2023), while the orange solid diamonds denotes the analysis of the spatially resolved young stellar population synthesis study (Sextl et al. 2024).