Unfortunately, even the President of the United States is not a fairy godmother who can simply wave a magic wand and make all the bad stuff go away. Biden can accomplish some of his immigration goals immediately, but others will take time to implement.
The skeleton of the US immigration law system is based on statutes passed by both houses of Congress. These statutes remain in force until they are repealed or modified by both houses of Congress, a prospect that may be unlikely if Republicans maintain control of the Senate. Statutes, however, provide only rough guidelines to immigration policy.
The next level down is immigration regulations. Immigration regulations must be based on a statute, and they cannot be inconsistent with the statute. Regulations typically provide more detail by “filling in the blanks” left by the vague statutory language. Regulations are passed by regulatory agencies such as the Department of Homeland Security, the officers of which are appointed by the President. Changing regulations typically takes several months.
Policies are essentially executive branch interpretations of the meaning of immigration regulations. Policies can be changed more quickly than regulations can. In fact, many of them can be changed immediately, although in some cases prudence dictates a period of preparation before any changes are finalized.