Tax law is housed in the internal revenue code AND implemented by the IRS.
LII ’23 [Legal Information Institute; April; Cornell University School of Law, “Internal Revenue Code (IRC),” https://www.law.cornell.edu/wex/internal_revenue_code_(irc)] pbk
The Internal Revenue Code (IRC) is the domestic portion of federal statutory tax law in the United States, and is under Title 26 of the United States Code (USC). The IRC has 11 subtitles, including income taxes, employment taxes, coal industry health benefits, and group health plan requirements. The implementing agency of IRC is the Internal Revenue Service (IRS).
IRC is the governing law of federal tax administration and collection. State law creates legal interests and rights, but IRC designates what interests and rights shall be taxed. Like criminal laws, IRC cannot be applied retroactively.