There are many many wonderful math books, and many many wonderful math resources on youtube. (As well as, of course, many many terrible ones!) I second AOPS! They have a fantastic series for younger kids called "Beast Academy" (https://beastacademy.com), and their challenge problem sets are fantastic! The same blog has a couple of other posts about them: https://leosstemhacks.wordpress.com/2019/11/06/the-three-bes...