Investing Demystified by Lars Kroijer Nice ~30 min introduction to basic index investing stuff, tends to be pretty concrete. Mostly focuses on the motivation of index investing and basic practice, and not so much of the mathematical theory. He advocates for a 2 fund portfolio (global stock tracker + global bond tracker). Only real criticism is that he exclusively recommends being completely globally diversified, whereas I'd argue some home bias is justified for most people. The rest of his youtube channel outside of this series can be pretty safely ignored.
The Bogleheads' Guide to Investing Pretty good general introduction to index-focused investing in general. Tends to be very broad in scope, meaning it probably hits some topics that mightn't be so relevant/necessary (e.g. b