I think I've seen you on the internet I mean on YouTube.
https://youtube.com/@mapachin_escenarios3?si=Ibm7Qby8V7CdHmY0
You have two options: go to mods folder and unzip mod.zip or download the game code which already comes unzipped.
Use blender to open our "meshes"(This is what 3D models are known as).
Yes, but they don't focus on helping the few projects like these.
There aren't many successful cross-platform open-source games.
https://github.com/vi3itor/GameplayFootball
This project has been abandoned several times. I don't even know who's developing it.