In Ancient and Modern China, most people have not owned a company, land or estate! Most people are employed by a company, and lease, particulary in Hong Kong!
In daily use, it's better to consider 所有 (pertaining ownership) as a two-word structure and 所有 (“all”) a single word. Now, if taken literally, it actually makes more sense that 所有 signifies ownership. 所 can be used before a verb to turn it into a noun, so 所有 is a noun version of 有(own). Nouns formed with this structure are often used as modifiers before or after other nouns, so 所有人 means “person that owns” i.e. proprietor, whereas 所有物 means “thing that is owned”.
The more interesting question really is how come 所有 means "all". As far as I can tell, there’s no consensus. I recommend those who are interested checking out this very detailed answer.
Yes. "所有" has the dual meaning of "all" and "posses/own". So, depending on the context, "所有人" can mean "all persons", or "the owner", the latter is often seen in legal matters to identify the owner of something. Fo example, "這房子的所有人是xxx" - "This house belongs to xxx", or "The owner of this house is xxx".
Other similar terms are "所有權", "所有權狀", and "所有物".