Bonjour Ann !
Nowadays, dessous is mostly used as an adverb, not as a preposition, meaning it's not followed with a location.
Its meaning is closer to underneath.
J'ai mis mon verre sous la table.I put my glass under the table.
J'ai mis mon verre dessous.I put my glass underneath.-> Here it's understood that the location has been mentioned previously, so is not repeated.
I hope that helps!
Bonne journée !
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