1.The answer to this questions is that снизу (synonym внизу) is an adverb, and can be translated into English as "below" or "at the bottom".
Сверху был крем, снизу - бисквит.
There was cream at the top and bisсuit at the bottom.
2.Под ("under" in English) is a preposition, used with the Instrumental case (meaning position, place) and with the Accusative case (meaning direction).
Под столом лежал ковëр.
There was a carpet under the table.
Положите ковëр под стол.
Put the carpet under the table.
3.We have just the same situation with cверху (вверху), "at the top", adverb and над, meaning "over", preposition.
Russian Language Teacher.