The most metal-poor stars (MMPs) are extremely rare objects located mainly in the Milky Way (MW) halo, representing the earliest generations of stars in the Universe. Studying stars with the lowest metallicities is crucial for understanding the masses of the first generation of stars, the various channels for the nucleosynthesis, and the assembly history of the MW. The synergy between Gaia data and high-resolution spectroscopy allows to dissect the Galaxy in a multi-dimensional space.

I will show a chemo-dynamical investigation of the MMPs focusing on the MW disc, The finding of a low-metallicity ”planar” population, detected in various surveys (e.g. Pristine, LAMOST), is at odds with previous expectations from simulations. This population strongly prefers a prograde and close to the plane motion, as the more metal-rich and younger disc. I will discuss the high-resolution spectroscopy (ESPaDOnS/CFHT) of a sample of prograde “planar” stars. The chemical analysis indicates that these stars formed in the same building block. Additionally, I will show that their chemical abundance distribution is less dispersed compared to MW halo stars at the same metallicity. I will discuss the multiple nucleosynthetic channels that are necessary to explain their chemical properties, i.e. neutron stars mergers, the range of supernovae II masses. I will also discuss how their presence is related to the early assembly, interpretating it with high-resolution cosmological zoom-in simulations.