  A search for new resonances decaying into a pair of jets is reported using
the dataset of proton-proton collisions recorded at $\sqrt{s}=13$ TeV with the
ATLAS detector at the Large Hadron Collider between 2015 and 2018,
corresponding to an integrated luminosity of 139 fb$^{-1}$. The distribution of
the invariant mass of the two leading jets is examined for local excesses above
a data-derived estimate of the Standard Model background. In addition to an
inclusive dijet search, events with jets identified as containing $b$-hadrons
are examined specifically. No significant excess of events above the smoothly
falling background spectra is observed. The results are used to set
cross-section upper limits at 95% confidence level on a range of new physics
scenarios. Model-independent limits on Gaussian-shaped signals are also
reported. The analysis looking at jets containing $b$-hadrons benefits from
improvements in the jet flavour identification at high transverse momentum,
which increases its sensitivity relative to the previous analysis beyond that
expected from the higher integrated luminosity.
