  The formation of the Galactic disc is an enthusiastically debated issue.
Numerous studies and models seek to identify the dominant physical process(es)
that shaped its observed properties. Taking advantage of the improved coverage
of the inner Milky Way provided by the SDSS DR16 APOGEE catalogue and of the
ages published in the APOGEE-AstroNN Value Added Catalogue (VAC), we examine
the radial evolution of the chemical and age properties of the Galactic stellar
disc, with the aim to better constrain its formation. Using a sample of 199,307
giant stars with precise APOGEE abundances and APOGEE-astroNN ages, selected in
a +/-2 kpc layer around the galactic plane, we assess the dependency with
guiding radius of: (i) the median metallicity, (ii) the ridge lines of the
[Fe/H]-[Mg/Fe] and age-[Mg/Fe] distributions and (iii) the Age Distribution
Function (ADF). The giant star sample allows us to probe the radial behaviour
of the Galactic disc from Rg = 0 to 14-16 kpc. The thick disc [Fe/H]-[Mg/Fe]
ridge lines follow closely grouped parallel paths, supporting the idea that the
thick disc did form from a well-mixed medium. However, the ridge lines present
a small drift in [Mg/Fe], which decreases with increasing guiding radius. At
sub-solar metallicity, the intermediate and outer thin disc [Fe/H]-[Mg/Fe]
ridge lines follow parallel sequences shifted to lower metallicity as the
guiding radius increases. We interpret this pattern, as the signature of a
dilution of the inter-stellar medium from Rg~6 kpc to the outskirt of the disc,
which occured before the onset of the thin disc formation. The APOGEE-AstroNN
VAC provides stellar ages for statistically significant samples of thin disc
stars from the Galactic centre up to Rg~14 kpc. An important result provided by
this dataset, is that the thin disc presents evidence of an inside-out
formation up to R_g~10-12 kpc.(Abridged)
