Basically there's 2 interpretations
1. The Materialist Neurochemical explantion
DMT is produced in the human mind naturally when we sleep, when you smoke DMT you basically force yourself into a dream state while being fully conscious
When we dream, we imagine very detailed worlds, encounter autonomous entities,, people tend to have similar dreams etc. But the difference is we are fully concoius, so everything feels *real* in a powerful and terrifying way.
So it makes sense that people's experiences on DMT are similar, as opposed to LSD for example, which is a foriegn neurotoxin and so highly unpredictable, DMT is something our brains already know how to process.
2. The mystical inter-dimensional interpretation
Reality isn't real, our brains are inter-dimensional computers which are connected to multiple realities and doing DMT allows us to tap into the infinite spiritual complexity of the univerese.
Perhaps both are true to some extent – maybe we are more sensitive to spiritual phenomena when on DMT but it's mostly just nuerochemical.
I lean towards the first possibility, but the second is still plausible. Impossible to now either way.