Deeper understanding of C++/JAVA

So I resumed practicing DSA recently. I mainly use C++ for this purpose. Few days back I saw a post which was kind of a roadmap and the first bullet point there was to get thorough with the language. I kind of agree with this and at some point I was meaning to understand c++ better, few level deeper. So thinking of taking a pivot for this purpose.

So c++ veterans of grapevine, need your recommendations for this. Open to books and video tutorial both. Not looking for syntax learning and all but how c++ works under the hood. I maybe drawing an inappropriate parallel with this but something like namaste javascript by Akshay sahni or you don't know js kind of material. I did started with gfg c++ blogs but that material feels unstructured and a bit less intuitive.

Grateful to you all in advance.