This doesn't answer my question. CSS-in-JS libraries are not magic that summons CSS out of thin air. They basically do the same thing Tailwind does: they watch

This doesn't answer my question. CSS-in-JS libraries are not magic that summons CSS out of thin air. They basically do the same thing Tailwind does: they watch your code for changes, they re-build/re-create raw CSS by, yes, often using string concatenation etc.