The video introduced in the video is here.
The University of Tokyo Entrance Examination: Kill the Difference between Extreme Values of a Cubic Function in an Instant [Math Good Questions Dojo #1].
⇨    • 【東大入試】３次関数の極値の差を瞬殺せよ【数学良問道場＃1】  
The Math Practice Test #1: How to solve a problem in a way that is easy to understand and easy to use!
⇨    • 【永久保存版】共通テスト頻出の積分テクニックを完全網羅！