  Supernova (SN) 1987A is the nearest supernova in $\sim$400 years. Using the
{\em JWST} MIRI Medium Resolution Spectrograph, we spatially resolved the
ejecta, equatorial ring (ER) and outer rings in the mid-infrared 12,927 days
after the explosion. The spectra are rich in line and dust continuum emission,
both in the ejecta and the ring. Broad emission lines (280-380~km~s$^{-1}$
FWHM) seen from all singly-ionized species originate from the expanding ER,
with properties consistent with dense post-shock cooling gas. Narrower emission
lines (100-170~km~s$^{-1}$ FWHM) are seen from species originating from a more
extended lower-density component whose high ionization may have been produced
by shocks progressing through the ER, or by the UV radiation pulse associated
with the original supernova event. The asymmetric east-west dust emission in
the ER has continued to fade, with constant temperature, signifying a reduction
in dust mass. Small grains in the ER are preferentially destroyed, with larger
grains from the progenitor surviving the transition from SN into SNR. The ER is
fit with a single set of optical constants, eliminating the need for a
secondary featureless hot dust component. We find several broad ejecta emission
lines from [Ne~{\sc ii}], [Ar~{\sc ii}], [Fe~{\sc ii}], and [Ni~{\sc ii}]. With
the exception of [Fe~{\sc ii}]~25.99$\mu$m, these all originate from the ejecta
close to the ring and are likely being excited by X-rays from the interaction.
The [Fe~{\sc ii}]~5.34$\mu$m to 25.99$\mu$m line ratio indicates a temperature
of only a few hundred K in the inner core, consistent with being powered by
${}^{44}$Ti decay.
