What is the practice people call "shamata practice". Where did this idea come from?
The custom of labeling meditation &quot;shamata practice&quot; probably originates from wrong or faulty practise, where excess or faulty suppression concentration causes calmness to predominate.
As for the original teachings attributed to the Buddha, 'samatha' is merely one of two results (the other being 'vipassana') of developing right concentration.

Thus for him, having thus developed the noble eightfold path... for
him these two qualities occur in tandem: tranquillity &amp; insight
(samatho  ca vipassanā ca).
MN 149

If we are not clear about what right &amp; wrong concentration are, Bhikkhu Buddhadasa offered his opinion:

As for samadhi, an empty mind is the supreme samadhi, the supremely
focused firmness of mind. The straining and striving sort of samadhi
isn't the real thing and the samadhi which aims at anything other than
non-clinging to the five khandas is micchasamadhi (wrong or perverted
samadhi). You should be aware that there is both micchasamadhi and
sammasamadhi (right or correct samadhi). Only the mind that is empty
of grasping at and clinging to 'I' and 'mine' can have the true and
perfect stability of sammasamadhi. One who has an empty mind has
correct samadhi.