Your post is not how things work, already built homes are grandfathered in. There are still houses with knob and tube wiring, balloon framed houses, all sorts of things that aren’t allowed any more by building code.

If a Toll Brothers/(any builder) has generic plan house blueprints that need to be updated to include the new code changes, they pay an architect to change the drawings to include the new code requirements, and then continue building that design of house.

This is an extremely exaggerated take. Again, the building codes do not upgrade on that fast of a time scale and even if a house is technically not up to code, they are grandfathered in based on the time they were built. A house that was built 10, 20, even 30 years ago, is still totally fine to live in, even if not up to the most current codes.