  A search for pair production of bottom squarks in events with hadronically
decaying $\tau$-leptons, $b$-tagged jets and large missing transverse momentum
is presented. The analyzed dataset is based on proton-proton collisions at
$\sqrt{s}$ = 13 TeV delivered by the Large Hadron Collider and recorded by the
ATLAS detector from 2015 to 2018, and corresponds to an integrated luminosity
of 139 fb$^{-1}$. The observed data are compatible with the expected Standard
Model background. Results are interpreted in a simplified model where each
bottom squark is assumed to decay into the second-lightest neutralino $\tilde
\chi_2^0$ and a bottom quark, with $\tilde \chi_2^0$ decaying into a Higgs
boson and the lightest neutralino $\tilde \chi_1^0$. The search focuses on
final states where at least one Higgs boson decays into a pair of hadronically
decaying $\tau$-leptons. This allows the acceptance and thus the sensitivity to
be significantly improved relative to the previous results at low masses of the
$\tilde \chi_2^0$, where bottom-squark masses up to 850 GeV are excluded at the
95% confidence level, assuming a mass difference of 130 GeV between $\tilde
\chi_2^0$ and $\tilde \chi_1^0$. Model-independent upper limits are also set on
the cross section of processes beyond the Standard Model.
