I looked it up on YouTube and pretty much found NOTHING. But people have made SDK’s for Unity so obviously it’s possible 
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Edit: If you have any tutorials or learning resources for this please lmk
Unity itself is an SDK, so what are you actually talking about? Please ask concrete and detailed questions that can be answered. What do you actually want to do?
“SDK” is a very broad and vague term which could mean a lot of different things.