[Need help to understand this effect]

https://demo.langoorqa.net/mylan/  I want to build something similar to this but the grid design should be random

[Need help to understand this effect]

See the Pen PoqxzMV by ZachSaucier (@ZachSaucier) on CodePen

 

I tried using this code and spend many hours as I am new to coding want to make my portfolio project that I am using this kind of background but with different size images gallery but with grid type of design where the height and width is not constant not able to solve this mystery Please let me know any way I can achieve this