  The analysis of the APOGEE DR16 data suggests the existence of a clear
distinction between two sequences of disc stars at different Galactocentric
distances in the [$\alpha$/Fe] vs. [Fe/H] abundance ratio space: the so-called
high-$\alpha$ sequence, classically associated to an old population of stars in
the thick disc, and the low-$\alpha$ sequence, which mostly comprises
relatively young stars in the thin disc. We perform a Bayesian analysis based
on a Markov Chain Monte Carlo method 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. 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 time-scales compared to the external ones. In the
outer disc (with radii $R>6$ kpc), the chemical dilution due to a late
accretion event of gas with primordial chemical composition is the main driver
of the [Mg/Fe] vs. [Fe/H] abundance pattern in the low-$\alpha$ sequence. In
the inner disc, in the framework of the two-infall model, we confirm the
presence of an enriched gas infall in the low-$\alpha$ phase as suggested by
chemo-dynamical models. Our Bayesian analysis of the recent APOGEE DR16 data
suggests a significant delay time, ranging from $\sim$3.0 to 4.7 Gyr, between
the first and second gas infall events for all the analyzed Galactocentric
regions. Our results propose a clear interpretation of the [Mg/Fe] vs. [Fe/H]
relations along the Galactic discs. The signatures of a delayed gas-rich merger
which gives rise to a hiatus in the star formation history of the Galaxy are
impressed in the [Mg/Fe] vs. [Fe/H] relation, determining how the low-$\alpha$
stars are distributed in the abundance space at different Galactocentric
distances.
