BepInEx is a universal Unity plugin framework which aims to provide a unified overlayed environment for different Unity games. It allows for runtime code modifications and has been widely used in modding various games.
Download the latest version that is compatible
Extract the contents to the root directory of Cities Skylines 2.
Run the game once to ensure that BepInEx initializes and creates its config folder.
This is not a standalone mod for Cities: Skylines II.
BepInEx is just a modding framework.