That was one of the more baffling things to try to explain to somebody.
That, and the patterns.
Amazing I lived through that, or more correctly, God kept me safe despite myself. Amazing Grace....
OH amen, amen, amen, AMEN ! ! !
First of all: events such as sounds, smell, taste, etc. are actually recorded events, or memories, within the brain. Those memories represent stored locations within specific/physical locations within the brain that are accessed by the brains wiring and routing system.
Normally, those memory locations are only associated with and accessed by very specific pathways and activity that is related to that region of the brain such as the sense of smell, the nose's sensory organs, etc. and a memory location of a smell or a similar smell.
Now if you bypass the normal wiring of the brain and enter that region of the memory and stimulate that area via another network path, then you will have the perception of smelling something that doesn't exist. Or hear something that doesn't exist.
What LSD appears to do is to open all pathways and allows parts of the brain that wouldn't normally communicate with each other to communicate and stimulate each other.