In Zen Buddhism one often hears that body and mind are one but in theory the the soul enters a new body after death rebirth. Are they seperated after all?

This question and this way of looking at things exists from the ancient past. You the questioner has already divided into two by asking this question. A European philosopher once said: Suffering is the division of mind.

A young man asked the Buddha about past lives, and this is how the Buddha answered. A man was hit by a poisoned arrow. He then goes on about where the arrow came from, who shot it, what type of arrow it was, and by doing so, the poison spread throughout his body and he died. The Buddha said: The suffering person right here needs to pull out the arrow, without having the need to know about the arrow. The Buddha only wanted to take the suffering from the people.

The two views, one which says that after death the soul will chose a new body and the other view which says that all disappears after we die, the Buddha disliked both views.

The reason is that the person being alive right here and now is only getting confused by these kind of ideas. The Buddha saw his responsibility in showing each person to not lose their own stance. What is much more of importance is that while we are living in this world, how can we together with other create a society free from suffering. For this we need to use all our energy.