In C++, how can one make it's game load content developped by other people?
For instance in Java you can look for jar files and load specific class from it.
There are 2 primary approaches to this:
Make your game open source. People have access to the source of the game and can make changes to the code.
Design a modular game. For the modules you want to allow people to mod, for example the Game AI, provide hooks. These hooks in C++ will either be in the form of dll calls (or COM, even) to user provided C++ implementations, or by a script, such as lua. You can allow people to introduce new content, such as textures, models, levels, etc by using standardized formats (e.g. png, fbx, json, xml), or specifying the proprietary format you use.
Further information, such as how to best design such dll interfaces are usually opinion based or highly dependent on specifics.