In my younger years I was very fascinated by malware, it's what got me into IT eventually. Back then I was active on some forums with marketplaces where one could easily buy and sell such services. I didn't do anything serious besides some slightly gray area stuff, but never sold or bought anything or compromised any unwilling victim. I did know some people who were later caught by the FBI and spent a long time in prison though.
In hindsight it's super crazy that this was a thing and probably still is.
About these operations, I honestly think they're not that spectacular even though they make it seem so. Anyone can buy a license for a random botnet for a couple of bucks and reverse engineer what's going on on compromised systems. I'm sure most of these botnets are hacked together pieces of junk code, which gathered a lot of installs through sheer luck and the fact that the FBI was looking away for a while.