Do you mean the kind of library you find in the File menu under Libraries? Or do you mean a JS library included with the source code?
For the former, in the repo, under libraries, you put the xml and then you edit LIBRARIES (a text file) in your favorite simple text editor (i.e. not Word).
For the latter, primitive blocks are listed in the code that generates the palette category it's in. Look in src/objects.js and search for your favorite block name that has either an unusual word ("inherit") or more than one word in a row ("keep items").