They get to a million gold by farming their own materials, not buying everything on the auction house, and selling excess items/materials on the auction house.
There isn't some magical method to getting tons and tons of gold, in reality it's just a matter of being frugal and not blowing all your money on stuff.
I've hovered around a million gold for a couple years now, but I dropped a bit because I wanted to buy character services without spending real life $. All the random mounts that cost hundreds of thousands of gold I've passed on for the most part, with only a few exceptions (namely, the Grand Expedition Yak that I pooled all my gold for in Mists of Pandaria and have never regret purchasing once).
If I want something, I'll work for it and get it because I didn't spend my money on everything else on the way. I'd imagine a good part of the gold people have for this dino are the same thing. But 5 million is a heck of a lot of gold, and not something people can get without really playing the market hard or selling lots of WoW tokens, or grinding to a virtually insane degree.
Personally I level up professions because I think it's fun. I don't make much gold from them. The only time I ever made gold from professions was during WoD with all the daily cooldowns where I was literally printing gold for just logging into my alts daily and turning crap in and pushing a button or two. It's been pretty rare for me to see non-consumable professions (i.e., alchemy, or jewelcrafting and inscription when they were useful) make people gold unless they are just selling items from obscenely rare recipes.
- - - Updated - - -
I mean I don't even need that. Since they boosted the amount of characters per server, I can just mail all my crap to a bank alt made just for this and auction it all together. I don't sit and watch the auction house 24/7 to adjust prices; I don't have the patience for that. I'll adjust the prices every day or two. Frankly, I'm fine with 75% of my items selling instead of 90% (excluding the obscenely expensive/rare items that take weeks to sell, or the ones that are just garbage and probably should be vendored anyways), since I can just relist them until they do sell.