It seems to me that physically we begin to feel pain in some place, for example, the heart.
I wonder how it is to die immediately and without pain, is it possible, I think that only in rare cases.
There is probably no exact answer to your question.
In a way I disagree. A person who is dying no longer feels pain, and that's because his organs have partially or completely shut down. In a few minutes after your heart stopped, your brain is still receptive. At least for a few minutes. This is when your whole life is flashing right before your eyes. Once your brain stopped receiving oxygen, then you're a goner.