  The results of a search for new phenomena in final states with $b$-jets and
missing transverse momentum using 139 fb$^{-1}$ of proton-proton data collected
at a centre-of-mass energy $\sqrt{s} = 13$ TeV by the ATLAS detector at the LHC
are reported. The analysis targets final states produced by the decay of a
pair-produced supersymmetric bottom squark into a bottom quark and a stable
neutralino. The analysis also seeks evidence for models of pair production of
dark matter particles produced through the decay of a generic scalar or
pseudoscalar mediator state in association with a pair of bottom quarks, and
models of pair production of scalar third-generation down-type leptoquarks. No
significant excess of events over the Standard Model background expectation is
observed in any of the signal regions considered by the analysis. Bottom squark
masses below 1270 GeV are excluded at 95% confidence level if the neutralino is
massless. In the case of nearly mass-degenerate bottom squarks and neutralinos,
the use of dedicated secondary-vertex identification techniques permits the
exclusion of bottom squarks with masses up to 660 GeV for mass splittings
between the squark and the neutralino of 10 GeV. These limits extend
substantially beyond the regions of parameter space excluded by similar ATLAS
searches performed previously.
