The compiler first builds an SSA IR tree of the word to be compiled (see compiler.tree.builder ) then this vocab converts it to a CFG IR tree. The result is not in SSA form; this is constructed later by calling compiler.cfg.ssa.construction:construct-ssa.
Each tree node type has its own implementation of the emit-node generic. In that word, cfg instructions (tuples prefixed with ##) are outputted to basic blocks and the cfg constructed.