To be more specific…all modern passenger aircraft have entry/exit and service doors on both the right and left sides. Almost without exception, each door or hatch has a counterpart directly across from it, for quick passenger evacuation in an emergency. However since the beginning of commercial aviation, doors on the left side have been used to passenger boarding, whereas their counterparts on the right side are either kept permanently closed, or used for galley and cabin service. That way, it’s possible to board or deplane passengers, and service the galleys and cabins simultaneously. Also (I am an ex-cabin crew member), occasionally, the right-side doors are used for unusual situations, such as boarding a wheelchair-bound passenger with a special lift, while not interfering with general boarding. Almost certainly having passenger entrance and egress via the left side is related to the captain being seated on this side, to allow him the best view of ramp and boarding activity.
One side note: a very few number of Douglas DC-3 airliners were actually made in the 1930s with only a right-side passenger door. Even photos of those are now a rarity!