Learn Turkish: Belediye çok çalışıyor, altyapı yenileniyor. - The municipality is working hard, the infrastructure is being renewed.

It happens in three steps. First, the adjective yeni (new) takes the suffix -le to become the verb yenilemek (to renew). Next, because the stem ends in a vowel, we add the passive suffix -n to make yenilenmek (to be renewed). Finally, we drop the infinitive and add the present continuous tense -iyor to get yenileniyor (is being renewed).

Since the infrastructure is receiving the action, why doesn't altyapı have the accusative object ending?

When we use the passive voice, the thing receiving the action becomes the grammatical subject of the sentence. Because altyapı is the subject of the passive verb yenileniyor, it stays in its bare dictionary form and doesn't take an accusative ending like -yı.

Is it common to link these two actions with just a comma instead of a word like ve (and)?

Yes, this is a very natural sentence structure in Turkish. When two actions are happening at the same time and are closely related, Turkish speakers often place them side by side separated only by a comma instead of using a conjunction.

How is the compound word altyapı built?

It combines alt (under or lower) and yapı (structure, from the verb yapmak). Just like the English word "infrastructure," it literally means the underlying structure.