Math Youtube Videos
A good introduction to machine learning is here, by Professor Yaser Abu-Mostafa at Caltech.
Some math channels that I recommend are 3Blue1Brown , Aleph 0 , Mathologer , Zach Star
Math Books and Journals for Undergrads
SIAM Review has an education section with a lot of papers that are easy for undergraduates to understand.
Proofs from THE BOOK has interesting proofs that do not require graduate-level mathematics.
The Professor Is In : The Essential Guide To Turning Your Ph.D. Into a Job is about finding a job after your Ph.D. A book that I regret not reading before I started my Ph.D.