How do I read multiplatform code or learn how to read multiplatform code?

Hello, I'm very new to this and I'm not really sure how to navigate the project directory of multiplatform code. Could someone please give me a run down of how to do so? Any suggested readings would be nice as well.

Also, it might be helpful to have an ARCHITECTURE.md file:
https://matklad.github.io/2021/02/06/ARCHITECTURE.md.html