There is quite a bit of overlap between using arrêter and s'arrêter. The cases where only the transitive form can be used are, of course, when there is a direct object, e.g., arrêter la voiture. The reflexive form would be plain wrong in this case.

When the focus is more on stopping and less on stopping something, often the reflexive form prevails.

In the text there is a direct object: I immediately stopped what I was doing. The COD being "what I was doing". Therefore the transitive form is needed in this case.

In that case pls explain why is s’arrêter being used in the sentence... les filles se sont arrêtées pour les regarder ....

referring to the exercise A day in the country side-B2

If there is no direct object (i.e., the object that is being stopped), and the object that's stopping is the subject itself, then use the reflexive form.

Thank You