In a library with multiple Content type assigned how can I choose the CT in the document if i'm using drag and drop? How can I change the CT in case there is a CT associated per default?
Drag and Drop is for only uploading the files into the documents library with minimal effort. It cannot help you to add meta data to the documents while doing so. You should be editing each file to add meta data to the document separately. Default content type will be applied while drag and drop.
Let me know if you have any questions.