A country is not a house. A much more realistic analogy, if you wish to compare political borders with some kind of privately-owned property, is a shopping mall. Are there border guards around a shopping mall? Unlikely. Does the shopping mall have a border? Yes.
Can you enter the “employee only” sections, or the manager’s offices, at will? No.
Security controls are applied where needed, at specific points. If the shopping mall is part of a “mixed use” complex, with apartments and offices, there will be varying sorts of access controls, depending on specific needs. If shopping malls were organized like prison complexes, few would shop there.
A general principle will be that if somebody invites person X, that person will be admitted, unless there is a specific objection to that particular person. For instance, if person X were a known murderer or shoplifter, that person X might be barred.
This principle is not even remotely consistent with the controls proposed by those who insist on sealing the borders, building walls, and so forth. Existing law preempts the right to travel; it denies the right of you or me to invite friends, employees, tenants, spouses, or anyone else to visit, work, or live with us. It arrogates those decisions unto itself. Whatever may be said about such policies, they are not libertarian; they are not consistent with a free country.