It depends a little bit on how comfortable you are with the platform and the technologies used in the codebase. Generally speaking you would look for the entrypoint into the application.

For Hockey Slam you can look for the entrypoint in Windows:

https://github.com/Mknsri/HockeySlam/blob/main/windows/win_m...

or Android:

https://github.com/Mknsri/HockeySlam/blob/main/android/app/s...

Then start tracing through, looking at any function calls that look like they contain anything interesting. Additionally in this project you can jump straight to the entrypoints of the game code:

https://github.com/Mknsri/HockeySlam/blob/main/game/game_mai...

Or the rendering code:

https://github.com/Mknsri/HockeySlam/blob/main/ogl/ogl_main....

Another way would be to look at the files and see if anything catches your interest, for example if you want to learn about the UI system, you could look at how a button is defined:

https://github.com/Mknsri/HockeySlam/blob/main/game/imui_sys...

Fabien Sanglard has great articles on studying classic video game engines, and could be used as a guide on how to approach codebases: https://fabiensanglard.net/quake2/index.php