My question is: does the Constitution apply to American Citizens only, or to all within the legal boundaries of the country? More specifically, the law-abiding within those boundaries?
Under the precepts of the Declaration of Independence, our rights come from God, not the State. Every person, citizen or not, has those rights.
Being in this country is a right for those born on our land, naturalized, or born to citizens abroad. For everyone else, it is a privilege.
Adjudicating a privilege is not in the same realm as adjudicating a right. The SCOTUS affirmed that the process can be truncated, if need be, for those abusing the privilege of being on American soil.
The Constitution refers to ‘persons’ so yes, but illegals by definition, remain under foreign jurisdiction. Because of this, they are not protected from deportation.
The 1996 Illegal Immigration Reform and Immigrant Responsibility Act specified that if a foreign national is within 100 miles of the border, has been here illegally for less than two years, and has not applied for asylum or citizenship, they can be summarily deported.
Newbies and those here longer than 2 years have more time to seek equal treatment by the court in determining eligibility for citizenship, but every foreign national is entitled to challenge:
Scalia, 1993, Reno vs Flores : “... the Fifth Amendment entitles aliens to due process of law in deportation proceedings.”
Millions were not citizens in the early days. Many took Oaths of Allegiance. My grandparents were not citizens. Only one of my grandfathers ever became a citizen. Neither of my grandmothers ever became citizens. They came to America about 1910.