1) Sous (under) is a preposition, dessous (down) is
an adverb. They have different functions in phrase.
Examples adverb vs. preposition
Elle va dessous.
She goes down. (not: Elle va sous.)
Mets le dessous.
Put it down there. / Put it under it. (not: Mets le sous.)
Va dessous.
Go down. (not: Va sous.)
2) Further there are variations of dessous, called pronominal adverbs, meaning that they can be used without related objects/verbs.
Examples
Il y avait un tas de papier là-dessous.
There was a lot of paper on it.
Ci-dessous on
voit la photo.
Down here, one can see a photo.
3) Is the object not vertically under another object, but more below/beneath it, dessous is used together with another preposition to take the function of a preposition.
