I think, Unicode says (please correct my if I am wrong), for example, Bangla can only use code points U+0980 to U+09FF. And also, codepoint X will represent letter Y.
Who specify which letter will be placed on what location in the keyboard? Is it also the job of unicode?
For example, Who specify which letter will be placed on what location in keyboard for Bangla? I am asking this because there seems to be a dozen keyboard layouts for bangla unicode Keyboards.
What is the deal here?
There is no unified standard or single standardization body for this. The individual layouts might not be standardized at all, e.g. developed by individual companies for their products, or be standardized by some national standardization body, governmental or non-governmental, for any give language/script.
This can also be seen in the Wikipedia list of some common Bengali keyboard layouts here that you are referring to in your question as well. For each layout the respective organization that developed/standardized them is also given. (Note that the site you linked is not actually Wikipedia, but a third-party interface to it.)