How did aircraft communicate during early air travel?
Radio was not widely used until the 1930s, before that, mostly light signals were used. Early ground support aircraft in WW I could drop little notes which were pencilled by the observer, stuffed in a small capsule to which a streamer was fitted, and that was dropped as closely as possible to the troops which should get that message. This method was still used in WW II, when most aircraft had radio, because only specially equipped ground troops had compatible receivers.

The first regular flights by night were 1921 for the air mail connection between Omaha and Chicago, and electric flood lights were positioned along the flight path.
When Lufthansa opened the first night connection between the Reich and East Prussia in 1926, they posted burning oil drums along the way to mark where the pilot had to fly. Communication the other way (air to ground) was not possible on those flights.

The first air-to-ground radio communication used morse signals, and this is the origin for these strange three-letter names for things like local air pressure at the ground (QFE) or pressure corrected to sea level conditions (QNH). There was a whole lot more of those, including some to order coffee before landing, but they are no longer necessary. Read all about it here.