To which regulating authority's regulations and standards are large sub-sonic transport aircraft (passenger or cargo) designed? The FAA's, or EASA's, or both?
The safe operations of an aircraft is called airworthiness. It is the standard by which an aircraft is determined fit to fly. Responsibility of airworthiness lies with national aviation regulatory bodies, manufacturers, as well as owners and operators. International standards are dictated by ICAO during the aircraft design process:
The International Civil Aviation Organization sets international standards and recommended practices for national authorities to base their regulations on
The answer to your question is:
The aircraft manufacturer makes sure that the aircraft meets existing design standards, defines the operating limitations and maintenance schedules and provides support and maintenance throughout the operational life of the aircraft.
In the USA, the FAA regulates airworthiness. For transport category aircraft, such as Boeing and Airbus, the regulations are in 14 CFR Part 25.