Is the water remaining on the left hand after Istinja’ (ablution after defecating or urinating) considered impure, and is it obligatory to wash the part of the body it touches, or does this fall under the category of Waswasah (obsessive whispering)?

The water from istinja' (washing after relieving oneself) that remains on the hand is considered pure as long as its characteristics have not changed due to impurity. Therefore, whatever it comes into contact with, be it the body or anything else, does not become impure.