Here is one medieval primary source that describes teeth care in detail. Some of the teachings of the 13th century Japanese Zen master Dogen Zenji were written down in a work called Shobogenzo. One of the chapters or "sermons" is called Semmen. The original title is 洗面. This translation, https://www.thezensite.com/ZenTeachings/Dogen_Teachings/Shobogenzo/055semmen.pdf rendered the title as "On Washing Your Face". The original was written around 1250.

It describes in detail how the monks should clean themselves, while also constantly using this mundane information as a Buddhist teaching, pointing to something much more profound. Still, the practical details are there. The translator's introduction in the link above points out that: "An implement for brushing the teeth similar to that which Dogen describes is still the preferred form of toothbrush with many Theravadin monks".

There is no mention of toothpaste, but a simple chewed twig suffices as a toothbrush.

Anyway, here is the part relevant to teeth cleaning:

Next, we use our willow twig [for a toothbrush]. [...] first we chew the willow twig. Then, holding the willow twig in our right hand, we recite the appropriate verse. [...] Having finished reciting this text, we chew on our willow twig.

The length of a willow twig is the width of four, eight, twelve, or sixteen fingers. As it says in Article 34 of the Code of Behavior for Members of the Greater Sangha, "We use for our teeth a twig of suitable dimensions. The longest is the width of sixteen fingers, the shortest is the width of four fingers." We need to keep in mind that it should not be shorter than four fingers and that beyond sixteen fingers is not a suitable dimension. The thickness is that of our little finger. However, there is nothing to preclude it from being thinner than that. Its shape is that of the little finger: one end thick, one end thin. The thicker end is chewed into fine shreds. As it says in the Great Scripture on the Three Thousand Forms of Everyday Behavior for Monks, "Do not go beyond a third of an inch along the length of the willow twig." We must chew it well, then rub and clean the front and back of our teeth as if we were polishing them. We rub and polish, and then rinse out our mouth from time to time. We should thoroughly polish and clean the base of our teeth above the gums and thoroughly scrape between our teeth, and wash that area clean. When our mouth is rinsed out again and again, our teeth will be washed clean.

After we have done this, we need to clean our tongue. [descriptions of how to scrape your tongue clean with the same willow twig]

Anyway, this is just one example, but hopefully it is detailed enough to be an acceptable response here.