  A search for resonances in events with at least one isolated lepton ($e$ or
$\mu$) and two jets is performed using $139 \, {\text{fb}}^{-1}$ of
$\sqrt{s}=13$ TeV proton-proton collision data recorded by the ATLAS detector
at the LHC. Deviations from a smoothly falling background hypothesis are tested
in three- and four-body invariant mass distributions constructed from leptons
and jets, including jets identified as originating from bottom quarks.
Model-independent limits on generic resonances characterised by cascade decays
of particles leading to multiple jets and leptons in the final state are
presented. The limits are calculated using Gaussian shapes with different
widths for the invariant masses. The multi-body invariant masses are also used
to set 95% confidence level upper limits on the cross-section times branching
ratios for the production and subsequent decay of resonances predicted by
several new physics scenarios
