Is the method of cleaning oneself after defecating by pouring water on the left hand and wiping the anus until the impurity is completely removed correct, knowing that the water does not reach between the labia with this method?

The best method for Istinja' (cleansing oneself after defecation or urination) is to remove what is on the excretory opening with a tissue or similar material, then wash it with water until the area returns to its rough texture as it was before the impurity. A strong presumption (ghalabat al-dhann) is sufficient for this; water is not required unless the impurity spreads outside the two excretory openings. If it does spread, then water is necessary to wash what has spread beyond them. It is also sufficient to rely on water alone.

The terms "Istitaabah" and "Istinjaa'" mean removing what exits from the two private parts from their openings, and this can be done with water or stones. Whereas "Istijmaar" is specific to using stones.

As for using the hand during Istinja' with water, it is permissible and there is no harm in it because it is for directly dealing with the impurity with the intention of removing it. It is recommended, in fact, to wet the hand with water before performing Istinja'.

The purpose of Istinja' is to remove impurity and cleanse the area. If it is strongly presumed that this can be achieved by the force of the water spray, then using the hand is not required. However, if it cannot be achieved except by using the hand, then it is necessary. Cleansing with water is achieved by restoring the roughness of the area through rubbing it, while continuously pouring water and relaxing slightly.