Aircraft manufacturers get a "type certificate" for a given type of aircraft which applies to all of the aircraft of that model that are made. The type certific

Aircraft manufacturers get a "type certificate" for a given type of aircraft which applies to all of the aircraft of that model that are made. The type certificate is expensive but gets amortized over all the aircraft produced. Changes (depending on how significant they are) can be covered by a "supplemental type certification" which leverages the underlying type certificate and only has to show that the changes are OK. Supplemental type certificates are much less expensive and is used to certify different models of a given aircraft (e.g. the 737-800 Max is a STC on a STC on a STC ... all the way back to the original 737 Type Cert[1]). There are also ongoing maintenance monitoring and updates that go into maintaining an aircraft's "airworthiness certificate". The manufacturer must monitor the fleet of aircraft it produces and produce maintenance instructions over the lifetime of the aircraft, including addressing unexpected fatigue and wear issues. Ref: https://en.wikipedia.org/wiki/Type_certificate https://en.wikipedia.org/wiki/Type_certificate [1] https://en.wikipedia.org/wiki/Ship_of_Theseus https://en.wikipedia.org/wiki/Ship_of_Theseus