any ideas what do cloud providers like pinecone user underneath ?

I'm from Pinecone. We use proprietary indexes. We could've used HNSW but decided the high memory consumption (ie, costly at scale) and slow index updates (ie, data gets stale) won't cut it for production use cases.

Oh I read a lot of HNSW stuff on your/Pinecone blog series. (Great learning resource btw, well done!) So I assumed you were using HNSW already. It's a news to me that you don't use it.