Does homeowners insurance cover all residents of the home?

No, such as 'roomates' and adult non-relatives. Refer to page one of your actual policy; Definitions. An insured means you and residents of your household who are; a. your relatives or b. other persons under age 21 in care of any person named above

Maybe this depends on your policy. The policies I've seen in California covered people with fire damage like this:

If you have a room mate that is actually paying rent, then no... they need their own renters policy.

If you have another adult non-relative (such as a live-in-lover) and they do NOT pay rent, then they are covered.