Does the presence of black impurities (dirt) emerging from the body after completing ghusl (ritual bath) invalidate it, despite thorough rubbing during the ghusl? And does the lack of complete cleanliness of the foot, after washing it three times, affect the validity of wudu (ablution) if dirt emerges from it when re-washing it?

Regarding the first question, if you immerse your body in water, the presence of dirt originating from the body does not cause harm, even if it has a substance that prevents water from reaching the skin, because it does not affect the validity of the ghusl (definition). This is what Al-Isnawi and Al-Baghawi mentioned. As for the second question, if the dust is light and does not prevent water from reaching the skin, then the wudu is valid. However, if it prevents (water from reaching the skin), like thick mud, then it must be removed. What the questioner mentioned seems to be of the first type.