The best source I know: https://betterexplained.com/
Also plenty of youtube channels, like Numberphile (many of the featured hosts have their own channels), 3Blue1Brown, Mathologer, Wrath of Math and many more. They have vast libraries covering pretty much any topic imaginable. It's all top tier presentation, so intersting they made me study math for fun - I'd rather watch Numberphile than Netflix.
I'm sure that's Fireship, without clicking the link.