brunoAjd to advance into higher mathematics a solid base in Algebra, Geometry and Trignometry is always assumed. Books don't go step by step into proving stuff because then it'd need a crane to move around.
If you find grasping the concepts hard from the textbooks you are following, I'd recommend you to skim through schaum's books once. They are -
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If you just wanna jump into the math, then -
For Calculus: [Login to see the link]
Linear Algebra: Gilbert Strang's youtube course (If you have time, I'd recommend to get a light introduction to Real Analysis. You know Sequence-Series, Convergence-Divergence etc etc)
Probability: Joseph K. Blitzstein's youtube video lectures
Statistics: [Login to see the link]
or if you want to just gobble up everything,all at once ----
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P.S - 1. All these recos assuming you want to take the ML route.
2. Although ISL & ESL are both excellent books, I'd ask you to get a good grasp of probability theory first before you dive into them.