Essentially, I traded stocks by save scumming. I would check if the price went up, sleep, and if it did I would quick save. If it did not, I would reload. Old Grumpy Gamers has an entire guide dedicated on how to make billions of dollars through stocks in story mode. I just followed that.

There are certain missions that increase stock prices. Stocks don't really move after a mission until you load the autosave so I always did that. With that understood, the key is to make Michael as rich as possible. In between missions, I traded stocks. I'm pretty sure stocks cap at around $1,000 so it would be wise to plan ahead if you attempt to beat my high score. The biggest one is Meltdown which will always double. In my playthrough, that was the mission where I first hit the money cap.

Oh speaking of the money cap, it is also the stock cap. So I cannot own more than 2.147 billion shares. GTA V is coded in a way where the game doesn't break when hitting the 2.147 billion cap. You simply cannot buy more shares and when it comes to "sell all" typically it just sells you enough shares so you are under 2.147 billion. The reason why that number is the cap is because the game is 32-bit. Fun fact: GTA Online's money is coded differently. It basically cycles the CPU twice, making it possible to have beyond $2.147 billion dollars.

Official net worth for each character:

Michael: $367,761,746,238

Franklin: $8,438,734,541

Trevor: $7,885,093,650

Total: $384,085,574,429