Resources to understand math better?

Hey guys, I do math, fm, physics and chemistry a levels and I want to study CS at cambridge or imperial. Math is a huge part of the admissions process so I want to be able to understand a lot of the math I am learning in my a levels to a very deep level. EG: understanding why derivatives give the gradient etc as many people have told me that having this ability gives you an edge over other people. Does anyone know any good resources like youtube channels etc that explains the things behind math in a simple yet informative way?
Thanks

Which board?

edexcel