A beachfront condo tower collapsed.

There are reports that it was being monitored because they discovered it
had sunk by about 2mm over a number of years.

Can satellites really be this precise ?

Is this a question of relative elevation compared to a known point in
the area? What sort of points can they use?

Did Shuttle SRTM also adjust the recorded data based on known elevation
of a point on ground and everything on SRTM data for that pass was then
made relative to that point? Or are they able to precisely calculate
elevation and shape of earth solely from the SRTM data collected by
Shittle (or other satellites that would do the same) without
correlation with a known point on ground as pint of reference?

In the case of the Shuttle SRTM mission, it was over a few days, so one
can go and measure elevation at those known points and waive at the
Shuttle as it flies overhead.

But if you notice a building has sunken 2mm over the years, how could
they do this? Vegetation grows. Beaches get "reconfigured" with every
storm. Ocean levels constantly change (waves, tides, storms), so where
can they get points of reverence? roads?