السلام عليكم If a disbeliver dies but never truly understood islam and only heard of it, is he to be entered into the hellfire? Or a better way to ask this is what exact conditions are there for a non-muslim who dies to enter into jahanum ?

The one whom the message of Islam genuinely reached and who refused it and died upon that is of the people of the Fire, and the one whom it never reached, or reached him only distorted with no way open to him to learn the truth, takes the ruling of ahl al-fatrah and is tested on the Day of Resurrection.