Let's say you have some sort of complex device that receives data from sensors, filters out less important parts and records more important ones. You only experience filtered signals.
Now imagine if device is put into some special state, in which filters are way less strict. Now more data gets behind filters. You now can experience data/information/signals that are discarded in normal states. Those signals are getting tagged as "important" instead.

Subjectively you will feel that you're getting lots and lots of important data. But it's not data - it is noise, that is normally discarded. This is why you cannot remember it or explain it afterwards.

P.S. I'm personally agnostic. There are explanations like "those insights just aren't transferrable from 6D hyperdimensional high vibration states to our 3D lower dimensions" and I'm ok with them. But I always have strong feeling that a lot of psychedelic experiences are explainable if you view it all as signal processing.