Learn Farsi: یک جرثقیل بزرگ در حال جابه‌جا کردن بتن است. - A large crane is currently moving the concrete.

In normal Farsi sentences, verbs go at the end. However, the formal progressive dar hâl-e construction turns the verb into a noun (the long infinitive). Because it acts like a noun, its object (beton) must follow it and connect with an Ezafe. You are literally saying 'in the state of the moving OF the concrete.'

When using a compound verb like jâ-be-jâ kardan in this structure, where exactly does the Ezafe go?

The Ezafe attaches to the very end of the infinitive. You treat the whole compound as one block, so the Ezafe goes on the helper verb kardan, making it dar hâl-e jâ-be-jâ kardan-e beton.

I learned to express ongoing actions using dârad (like dârad jâ-be-jâ mikonad). Why does this sentence end with ast?

Using dârad is the common colloquial way to form the progressive tense. This sentence uses the formal written equivalent: dar hâl-e (in the state of). Because dar hâl-e acts as a descriptive phrase for the subject, the actual main verb of the sentence is just the 'to be' verb ast at the end.

How does the compound verb jâ-be-jâ kardan break down literally?

The word jâ means 'place.' The phrase jâ-be-jâ literally means 'place to place.' When combined with the light verb kardan (to do/make), it creates a compound verb that means 'to move,' 'to shift,' or 'to relocate' something.