You don't just pop up trees in your game! every game has its own art style! I use Speedtree myself! but I moved on to a free open source version program called Tree it! https://80.lv/articles/tree-it-a-free-tree-generator/ those libraries are cool but usually, you make your own according to your art style! Witcher 3 also has its own specific version trees it's not like other trees, BTW there are lots of optimization and collusion that need to be adjusted before importing to a game engine, these things are not cheating! it's called the evolution of programs...oh BTW they don't just make it by hand even if there was no app like speed tree for it! there are lots of programmers created their own tree creation tools like the one for Blender Adds-On, or there is a script for 3Ds Max that makes it happen, you can find it at script spot, oh there is even a plugin for Zbrush! Maya... In Houdini it's much much better and easier since it's procedural, they used Houdini for the Last Ghost Recon game a lot! and there is also Plant Factory! so the point is programmers are smart! they find a way to make tedious things automatic and it's not cheating or anything!