Learn Farsi: این خودرو نه تنها خودران است، بلکه خیلی سریع هم است. - This vehicle is not only autonomous, but it is also very fast.

The first clause uses na tanhâ (not only) before the first adjective, khodrân. The second clause starts with balkeh (but) and places ham (also) right after the second adjective phrase, kheyli sari', just before the final verb.

Is it necessary to repeat ast at the end of both clauses?

It is grammatically correct and emphasizes both traits, but in everyday Farsi, the first ast is often dropped to sound more natural, leaving: in khodro na tanhâ khodrân, balkeh kheyli sari' ham ast.

What are the parts that make up the word khodrân?

It is built from khod (self) and rân (the present stem of the verb rândan, which means to drive). Put together, it translates literally to self-driving.

Could I use vali instead of balkeh to mean but in this sentence?

No, you must use balkeh when pairing it with na tanhâ (not only). The words vali and ammâ are used for simple contrasts, whereas balkeh is required after a negative phrase like na tanhâ to mean but also or but rather.