What you're describing is more like options, but futures and options both work in a similar way to what you described, but futures have an underlying commodity that you have an obligation to take possession of (such as barrels of oil) options on the other hand give you the right to buy the underlying stock, but you have no obligation to do so.

So if I have a futures contract for 1000 barrels of oil for $10 a piece, when that expires, no matter what the price of oil, $10000 will be taken out of my account and someone will contact me to come pick up those barrels (there's some nuance here, but let's ignore that)(funny story about this at the bottom). If I have an 10 options contracts for 1000 shares of stock XYZ at $10 a share and XYZ is at $9 a share, I can just let the contract expire worthless.

From here on out, I'm going to talk about options, because it's closer to what you are asking.

> I don't have enough money sitting around to buy those 100 shares, so let's say XYZ is trading at $200 by Jan 1st 2021, That means I have a contract where I can buy 100 of them at $100 and immediately sell them for $200, so in theory it's a $10k profit.... but because I don't have enough money to actually do that, do I just sell my future for something close to 100* $200 (because someone with enough money will buy it and do the actual trade?)

Contracts are exercised after hours. If you don't have the money you will usually collect the shares and be put in a margin call (you owe your broker money) and then the shares will be sold first thing the next day to cover the margin call. Some brokers will try and sell your contract for you before the end of the day the contracts expire if you don't have the money. So you may only get $9950 or something for your contract and it will be sold to someone else a few hours before the market closes.

> What happens if it's already well over $100 long before Jan 1? Can I just set a price and sell it whenever I want like a can with a regular share?

Yes, the contract itself cost money and can be bought or sold.

This isn't a perfect analogy, but think of the contract like a coupon. If I have a coupon for to buy a TV for $100 and the cheapest anywhere is selling that TV is for $200. That coupon has an intrinsic value of $100.

The analogy breaks down, because in the real world, you probably couldn't get $100 for that coupon. You'd probably get slightly less than the price difference. With options on the other hand, you usually pay slightly more than the price difference, because of the volatility of the underlying. Basically, someone might pay you $105 for that coupon because they think they the TV price will go up and they can sell it to someone next week for $110.

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* As a side note, when people were saying oil prices went negative last week, it's because the May futures contracts were about to expire. Basically, it was the second to last day people could trade the contracts before they had to take delivery of the oil and, since so few people are using oil right now, many of the places where people would normally store oil are full. Since you can't just dump oil down the drain, people holding oil contracts were willing to pay other people to take over the contracts so they didn't have to take delivery of the oil.