I made a video editorial for CF1923E : Count Paths discussing the DP solution that acts as the foundation of the small to large trick.
Youtube Link : https://youtu.be/D454rHDUb6E?si=FtfBwoKG3_p-8m7f
I also created practice contest with easy problems:
How do you learn these advanced algorithms and techniques
I wouldn't say it's an advanced concept as such. It's standard DP on Trees, which can be learnt from resources such as CP-Algorithms, USACO Guide and CF Blogs.