Under can be Adverb or Preposition, When we used under as a Preposition we use under when we talk about something which is below or lower than something else:
The cat is Under the table.
His slippers are Under his bed.
When we use under, we mean that something is covering or touching, we do not use below in this way
The wreck of the Yacht still remains under the sea. (We do not use Below in this sentence)
He had Hidden my slippers under the Bed. (Not below the Bed)
We don’t use under to refer to something in a lower position than something else.