How In The Hell Does One Get 600 cs?
Never missing last-hits, clearing nearby jungle camps between minion waves, having other lanners build balls of CS for a carry.
Some pro teams run a sudo-support champion like Nautilus top-lane and give up CS to a carry, which can be devastating when done right. Having +1 major item on your item-dependent champion can offer a significant advantage over an opponent.
In most cases getting crazy-high CS is due to a team-wide strategy on giving one particular player crazy high CS, often with the jungler and top-lanner donating some of their CS.
Max minion CS is pretty high if you never miss anything as well. 10, 20, 30, 40, 50, 60 minutes is 107, 234, 361, 487, 614, and 740 CS, respectively.
If a game drags on long enough you can also get crazy CS scores.
Once you pass 600 you're basically showing off for bragging rights at the detrement of your teammates though, seeing as that's max-build with nothing to spend any additional gold on.