Learn Russian: Закончив важный проект, мы поняли, что заработались. - Having finished the important project, we realized that we had gotten carried away working.

This specific за-...-ся pattern indicates absorption or excess. It means the subject got so carried away or deeply involved in an action that they lost track of time, or did it until they were exhausted. Here, it means we got overly absorbed in our work.

Is заработались considered a perfective or imperfective verb here?

It is perfective. The prefix за- makes the verb perfective. Even though работать (to work) describes a continuous process, заработаться focuses on the end result of that process: the completed state of having become completely absorbed or overworked.

Why use the form закончив instead of a phrase like когда мы закончили?

Закончив is a verbal adverb meaning having finished. At the B2 level, you will see this often in written Russian. It connects a background action to the main subject (мы) more concisely and elegantly than using a whole new clause with когда.

How does the ending of закончив tell us the exact timing of the action?

The suffix -в attaches to perfective verbs to form a past verbal adverb. Because it is perfective, it explicitly tells us the action of finishing the project was fully completed before the main action of realizing (поняли) happened.