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.

I don't believe them. I don't concern myself with figuring out their motivation. They aren't worth it.

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

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.