Andy Pavlo from the CMU Database Group has excellent courses on Database design on YouTube. Large time commitment, but he goes into great detail about internals

Andy Pavlo from the CMU Database Group has excellent courses on Database design on YouTube. Large time commitment, but he goes into great detail about internals and design tradeoffs, and is very knowledgeable about which database systems use which approaches. The intro course is about traditional databases, while the advanced course is about modern in-memory databases.