Purely player created. They're basically guilds in other games.
There are some built-in 'NPC Corporations' that you start on, and default back to if you get kicked out of a Corp, but they're basically just placeholders.
It's also important to acknowledge that these NPC corporations have a high tax rate so there's a financial incentive for players to leave these NPC corporations for Player driven ones. Most player corporations have a lower tax rate, less than 10% or matching if there's a war on. Sometimes a corp will increase taxes to recoup war bonds but it's only temporary.
There are some built-in 'NPC Corporations' that you start on, and default back to if you get kicked out of a Corp, but they're basically just placeholders.