See: The Code of Federal Regulations (CFR) Title 26 Internal Revenue.
Congress doesn’t micromanage. They pass laws that give broad authority to agencies and departments to create administrative law. This is why EPA regulations are law. The IRS has authority from congress to create the forms we use and formulate certain rules. I’m not well versed in tax law so I can’t be more specific, but there is something to be said for complaining to government agencies.
Whenever they change a regulation, the proposed change is opened up to public comment, for one thing. There is a literal newspaper-format document that comes out five days a week to update the CFR it’s changed so regularly. Congress can certainly decide to override, overrule, or change any regulation as it sees fit, but the IRS does the day-to-day decision making.
