first of all yes, he does use that brush but not as much as you think.>Also make sure your wacom/pen settings are on firm or harder because they control opacity with the pen and not manually unless they are glazing color
they dont touch those settings and ive comfirmed it from one of his vids its set at the default.>have a soft edge on one side of the brush.
true but again not as often as you think
ruan jias has 4 basic ways to build brushes. A soft brush with opacity+flow turned on. A wet brush being an opacity+flow brush. A hard brush with no opacity but can have flow enabled or disabled. and a dry brush with opacity on and getting different texture measurements with either texture or the dual brush option or both. he has one video where he shows us this.