  This paper presents the reconstruction of missing transverse momentum
($p_{\text{T}}^{\text{miss}}$) in proton-proton collisions, at a center-of-mass
energy of 13 TeV. This is a challenging task involving many detector inputs,
combining fully calibrated electrons, muons, photons, hadronically decaying
$\tau$-leptons, hadronic jets, and soft activity from remaining tracks.
Possible double counting of momentum is avoided by applying a signal ambiguity
resolution procedure which rejects detector inputs that have already been used.
Several $p_{\text{T}}^{\text{miss}}$ `working points' are defined with varying
stringency of selections, the tightest improving the resolution at high pile-up
by up to 30% compared to the loosest. The $p_{\text{T}}^{\text{miss}}$
performance is evaluated using data and Monte Carlo simulation, with an
emphasis on understanding the impact of pile-up, primarily using events
consistent with leptonic $Z$ decays. The studies use $140~\text{fb}^{-1}$ of
data, collected by the ATLAS experiment at the Large Hadron Collider between
2015 and 2018. The results demonstrate that $p_{\text{T}}^{\text{miss}}$
reconstruction, and its associated significance, are well understood and
reliably modelled by simulation. Finally, the systematic uncertainties on the
soft $p_{\text{T}}^{\text{miss}}$ component are calculated. After various
improvements the scale and resolution uncertainties are reduced by up to 76%
and 51%, respectively, compared to the previous calculation at a lower
luminosity.
