Complex sentence: By December 1918, Holt Thomas claimed in an advertisement that Airco was the largest aircraft company in the world and was building aeroplanes, engines and propellors in large numbers, as well as airships and flying boats.
Split into simpler sentences: By December 1918, Holt Thomas was able to advertise that his was the largest aircraft company in the world. His companies built aeroplanes and their engines and propellors in large numbers, and also built airships and flying boats.