I've just been made aware that some countries have bilateral agreements with the US. What this means is that you can stay in certain countries for an additional 60-90 days after having spent 90 days via Schengen. But only in that country. It's not automatic and you have to request it when entering the country. The immigration official will then update your file.
Here is some info from the French government. Has anyone done this?
https://us.diplomatie.gouv.fr/en/gradual-implementation-entryexit-system-ees?
From what I am reading this is true in:
Belgium (3 extra months)
Denmark (3 extra months)
France (90 extra days)
Hungary (90 extra days)
Italy (3 extra months)
Latvia (90 days in any half-year period)
Netherlands (90 extra days)
Norway (90 extra days)
Portugal (60 extra days)
Spain (90 extra days)