I would strongly advise against people learning based on LangChain.

It is abstraction hell, and will set you back thousands of engineers hours the moment you want to do something differently.

RAG is actually very simple thing to do; just too much VC money in the space & complexity merchants.

Best way to learn is outside of notebooks (the hard parts of RAG is all around the actual product), and use as little frameworks as possible.

My preferred stack is a FastAPI/numpy/redis. Simple as pie. You can swap redis for pgVector/Postgres when ready for the next complexity step.