From the Document: "Immigration is a multifaceted policy issue that involves temporarily or permanently admitting foreign nationals (in statute, 'aliens') into the United States and enforcing immigration laws. It is a perennial topic of congressional oversight and legislation. In recent years, the executive branch has taken the lead in implementing numerous policy initiatives, invoking discretionary authority conferred by statute."
CRS In Focus, IF10520
Congressional Research Service: https://crsreports.congress.gov/