Below are agencies that impact and address immigration and citizenship significantly. Their websites have both practical information as well as policy materials and some research by the agencies.
The Code of Federal Regulations (CFR) is arranged so that the titles correspond with the United States Code (USC) (Title 6: Domestic Security of the CFR parallels Title 6: Domestic Security of the USC). Federal Agencies, which are established by Congress put forth regulations, which when published in the CFR have the same force as law.