[xenadu02]

That's true. Aircraft that do not cross borders are only subject to the home country's laws.

FWIW I'm very glad in the US we still allow experimental and home-built aircraft. That's how we got aircraft in the first place and many experimental planes (most?) are built from kits engineered to similar specs as certified aircraft, they just don't go through certification.

However EASA does not actually make Boeing go through a full from-scratch certification again. For the most part EASA reviews the FAA's certification and does not actually put the 737 through a from-scratch certification. This is part of the various "Bilateral Aviation Safety Agreements".

For certain hard requirements (as you noted) there may be disagreement on the rules and the manufacturer may be required to do additional certification with FAA or EASA for the differences but again it is not usually a from-scratch process - it only covers the differences. Technically other countries have similar processes and their own agreements but FAA (being the origin country of powered flight and home to the most aircraft manufacturers) and EASA (home to Airbus and also highly involved since the early days of powered flight... the Wright brothers toured France with their flier to much fanfare, disproving a lot of skeptics in the process) are the leaders and everyone else mostly just follows them rather than inventing their own rules and certifications.