https://www.reddit.com/r/learnmath/comments/kkyx5/can_somebody_explain_fubinis_theorem_to_me/
Abother good explanation, in the last line we sum over every small bit of of f(x) i.e. integrating https://math.stackexchange.com/questions/549923/how-the-product-of-two-integrals-is-iterated-integral-int-cdot-int-iint