Do practices like Zazen have any basis in Sutras? AFAIK, there are not sutras that describe the various forms of meditation that are available nowadays. Only in some Pali Suttas are some meditations vaguely described (especially pertaining to Jhanas and Satipatthhana).
Was meditation ever described in Sutras/Suttas? If not, what is the basis of Zazen practice? Or was it supposedly transmitted by word of mouth by the Buddha to his disciples which did not end up becoming part of the canon?
The posture, the tongue touching the palate, the hand mudra etc - I know these are essential from practice - are not described in any Sutras I have read so far. Where did it come from?
Depends on what you mean by "zazen". For one who reads Chinese (or Japanese, no doubt), the question is, "Do practices like 坐禪 have any basis in sutras?" Of course. A large number of sutras talk about 坐禪 = sitting meditation. This does not specify a particular technique or method, just sitting meditation in general.
Much of this material is found in material in Chinese that comes from the northern Indian traditions, travelling through the silk road into China. These parallel the Pali texts, but are from different schools. Not much is available on these in English. So, people only look at the Pali and miss the connections.
But, if you are talking about zazen only in the sense of that taught by Dogen Zenji in the Japanese Sotoshu Zen tradition, then there are certainly some differences.