Context.The analysis of the latest release of the Apache Point Observatory Galactic Evolution Experiment project (APOGEE DR16) data suggests the existence of a clear distinction between two sequences of disc stars at different Galactocentric distances in the [α/Fe] versus [Fe/H] abundance ratio space: the so-called high-αsequence, classically associated with an old population of stars in the thick disc with high average [α/Fe], and the low-αsequence, which mostly comprises relatively young stars in the thin disc with low average [α/Fe].Aims.We aim to constrain a multi-zone two-infall chemical evolution model designed for regions at different Galactocentric distances using measured chemical abundances from the APOGEE DR16 sample.Methods.We performed a Bayesian analysis based on a Markov chain Monte Carlo method to fit our multi-zone two-infall chemical evolution model to the APOGEE DR16 data.Results.An inside-out formation of the Galaxy disc naturally emerges from the best fit of our two-infall chemical-evolution model to APOGEE-DR16: Inner Galactic regions are assembled on shorter timescales compared to the external ones. In the outer disc (with radiiR > 6 kpc), the chemical dilution due to a late accretion event of gas with a primordial chemical composition is the main driver of the [Mg/Fe] versus [Fe/H] abundance pattern in the low-αsequence. In the inner disc, in the framework of the two-infall model, we confirm the presence of an enriched gas infall in the low-αphase as suggested by chemo-dynamical models. Our Bayesian analysis of the recent APOGEE DR16 data suggests a significant delay time, ranging from ∼3.0 to 4.7 Gyr, between the first and second gas infall events for all the analysed Galactocentric regions. The best fit model reproduces several observational constraints such as: (i) the present-day stellar and gas surface density profiles; (ii) the present-day abundance gradients; (iii) the star formation rate profile; and (iv) the solar abundance values.Conclusions.Our results propose a clear interpretation of the [Mg/Fe] versus [Fe/H] relations along the Galactic discs. The signatures of a delayed gas-rich merger which gives rise to a hiatus in the star formation history of the Galaxy are impressed in the [Mg/Fe] versus [Fe/H] relation, determining how the low-αstars are distributed in the abundance space at different Galactocentric distances, which is in agreement with the finding of recent chemo-dynamical simulations.