If someone really want to excel in whiteboard interviews, stop watching tutorials and start practicing algorithms(may be just your own thought process). There are a hell lot of online coding sites and competitions round the globe, jump over them and practice a lot. Don’t just solve it, come up with the best solution. And don’t directly wirte codes for a problem. 60% of the problems can be solved mathematically without looping for n times.
Practice! Practice! Practice!