Under as a preposition can mean
1 - in a place which is directly below: he hid under the table; the coin rolled under the piano.
2 - less than: she is under thirty; he is under age; it was sold for under $100; he ran the mile in under four minutes.
Below as an adverb means lower down: he stood on the hill and looked down into the valley below.
As a preposition, it means lower than: the temperature never goes below 25?; if you look below the surface; you shouldn't have hit him below the belt; his marks were considerably below (the) average.
Underneath as a preposition means under/beneath: she wore a long woollen cardigan underneath her jacket; he was sheltering underneath a chestnut tree.
As an adverb it also means under: he was wearing a thin shirt with nothing underneath.
Beneath as an adverb means underneath/below: he looked out of the plane at the mountains beneath.
As a preposition it means under: have you looked beneath the cooker?
He thinks it is beneath him, he thinks it is too insignificant/too unimportant for him to deal with.