It can be hard to know where to start with some of these concepts, especially so given that a lot of recent developments (e.g. RAG) are developing so rapidly that there’s unlikely to be a reference book you could refer to anytime soon that would be current.
That said, I do find that documentation is getting better depending on where you look. The documentation for higher level tools like LlamaIndex is a good starting point for understanding the concepts (not so much in terms of explaining the concepts, but showing where they fit into the overall picture, then you can deep-dive elsewhere on the different parts).
YouTube has always been a mixed bag of very little solid information in a sea of non-experts trying to attract clicks for the latest trends, so it’s not a great starting point IMHO.