Map modding allows you to add new maps and missions to the game. Advanced Unreal knowledge recommended, the modding documentation will only tackle Bus Simulator 21 specific topics. For general Unreal question please consult their documentation.
Use the Map_ModMap Plugin as starting point for your map. The base setup has already been done in that map, which allows you to extend it to your liking. It is recommended to rename the Mod Map level to prevent load conflicts if players install more than one map mod.