Yes, occasionally a group of existing law will be assembled together and approved as a block (this is how the U.S. Code is formed), and then they can tweak with

Yes, occasionally a group of existing law will be assembled together and approved as a block (this is how the U.S. Code is formed), and then they can tweak with USC from there directly. Other times they pass the law and it just kind of floats out there, in the "Statutes-at-Large" until some poor legislative assistant decides to/gets told to go consolidate the groups back into appropriate titles of USC (which requires another bill in Congress). Even when you're making a law which is basically just a diff to the U.S. Code though, the various interactions of each of the changes can be difficult to discern the real meaning of. But on the other hand it's nice to be able to simply fix the broken part of a law without having to do a full rewrite.