I’ve read that, as the brain shuts down, it is flooded with a chemical called DMT, which creates a euphoric and other-worldly feeling.

We dont know exactly, but the current working theory is that the brain pulses extremely strongly when dying, which produces extremely vivid images, including the famous “light at the end of a tunnel”.

Something I heard is that peripheral visions dies quicker than central vision for the tunnel aspect. I should probably verify it though