Why did Buddha die as a normal human being ?

It is mentioned in some articles that Buddha died from eating poisonous mushrooms.

If he was enlightened, why is it that Buddha could not separate his soul from the body through meditation to enter Nirvana ? Didn’t he have the ability to sense that the mushrooms were poisonous? Yogis claim that they are able to separate the soul from the body and end their life on earth thus ending the cycle of rebirth. If the gods watched as Buddha was being served poisonous mushrooms and Buddha himself could not sense this, how can we know he was enlightened ? If a person such as Buddha who gave up his normal life, stopped eating meat and meditated for so long with the hope of entering Nirvana, why did the gods/universe or Buddha himself, need poisoned mushrooms to end his life ? Is the human soul just a database ( like computer software ) that is being used by Extraterrestrial/Alien beings so that they can absorb all the information at death and transfer/sell it to another being ? If that’s is the case, is there a Nirvana or Heaven at all ?