Learn Farsi: آنان در حال بازسازی یک مجتمع تجاری هستند. - Those people are currently reconstructing a commercial complex.

Because this sentence uses the formal progressive structure. Instead of the conversational dârand ... mikonand, it uses dar hâl-e (in the state of) followed by the verbal noun bâzsâzi (reconstruction).

Why is there an invisible Ezafe connecting the action to the object (bâzsâzi-ye yek...)?

In the dar hâl-e structure, the action is expressed as a noun (bâzsâzi). Because it acts as a noun rather than an active verb, the thing being rebuilt connects to it with an Ezafe, translating literally as 'the reconstruction of a commercial complex'.

Is there a reason ânân is used here instead of the more common ânhâ?

Yes, ânân is a more formal, literary word for 'they' or 'those people'. It pairs naturally with the dar hâl-e progressive structure, which also belongs to a higher register of formal or written Farsi.

How is the verbal noun bâzsâzi formed?

It combines the prefix bâz- (meaning 'again' or 'back') with sâzi (building or making, derived from the verb sâkhtan).