As others have eluded to here, I've always felt that it was a combination of the brain shutting down and firing off memory fragments or re-experiencing the overall tone of their life. In other words, if they led a mostly happy, good and honest life, they would be more likely to experience something happy or positive (Heaven) and conversely, if they led a mostly sad life or a life filled with hate or anger then they would be more likely to experience something sad, scary or negative (hell).

I have had lucid dreams in which I "knew' that I was actually at a particular location. l could see in sharp detail all that was around me. I could say that I had seen "Heaven"", but it was only a dream.

The minds sees what the mind wants most especially in dreams or near death experiences I think heaven was just a way of getting people to fight in wars without having to look back at what they've done or lost.

Hallucinations.

I saw heaven on the first orgasm with my soon-to-be wife.
I saw hell after I married her.

too much acid or they want money

You are probably talking about "near death experiences." Those are ,in reality, nothing more thyan the manifestations of the brain beginning to shut down. Those manifestations are fed by the core beliefs that have been programmed into the person's mind.

When you are faced with a question, such as does heaven (or hell) exist? Providing that you care enough about the subject matter you will adopt one of two thinking processes. You will either A care about the truth, or B care about what you want to be true. Based on any of the arguments that I have heard, anyone claiming to have experienced something supernatural, such as heaven, hell, ghosts, pyschic ability etc, is probably a person B rather than a person A

They are, and its crazy.

Anoxia in the brain produces hallucinations which some people refer to as Near Death Experiences involving visions of Heaven or Hell...

[neuroskills.com]

There are many non-ordinary cognitive states that even modern science doesn’t yet fully understand which ancients could have described as heaven or hell. I have “visited” some of those “places” but would prefer to describe them in naturalistic terms rather than envisioning them as supernatural.

It wouldn't be the first time someone used a religious scam to make a buck.

They're delusional.