You even see them on leaderboards and they are crushing it.
The first thing they do is they test all sorts of places to see which ones get them the best results and which ones don't. Once they know what works, they scale.
But then, they never share with others and here's why:
If they do, they will create MORE COMPETITION for themselves which is a BAD thing. If you market somewhere online and you are getting good results, it will not be smart to share with others.
Buy traffic, test it out on a very small scale, and if you are not getting results with it, don't use it anymore. Make sure you write it down so you keep it for future reference. Same thing with traffic that does convert.