>>> it is declared as uuid. But how does it occupy only 16 bytes?
>> Because a UUID is internally simply a 128bit number - the dashes you see are just formatting.
> Sorry if this is silly but if it is a 128 bit number, why do we need 32 characters to represent it?
The 36 (or 32 without the dashes) characters are just the default hex representation.
If you wanted to, you could convert it to a bigint (or a numeric).
See e.g. here for an example: https://stackoverflow.com/a/27286610