Architectural/algorithmic tweaks do advance the efficiency frontier nicely. But raw intelligence mostly comes from data (not just its sheer quantity, but also how it's curated & cleansed) and the scaling law. The know-how about data curation doesn't seem to get published much, even among the open-weight labs, though.

This. Even in the efficiency frontier, it is a lot of data curation that actually makes many of those tweaks actually work at scale in practice.