Ustādh, when should the Siwāk be used as it relates to the Ṣalāh? And should it be used before voluntary Salah’s? If so then let’s say I’m about to pray the Sunnah of dhuhr do I use Siwāk between the two rakahs then again before the Fard and again for the two rakahs after

Use it at wudu, and again when you stand for the prayer before the takbir, and this is for every prayer, fard and nafl, because the Prophet ﷺ said, were it not that I would burden my Ummah I would have ordered them with the siwak at every prayer (agreed upon). Its place is the entering of the prayer, not between the two rakahs of it.