What will happen if I call the loan company and tell them to come and take my car, I can't afford the payments anymore?

Your credit will be shot. Not a big deal if you don't borrow money.

However, they will sell the car for less than its worth and you will legally owe the difference.

If you owe 20k and they sell for 10k, you will owe the balance.

yes, to make it in simple terms, if you owe 15,000 right now, and perhaps the car now used is worth about 14,000. They will come and get the car ( a repo) and they will add a fee for repo, and legal fees for doing the legal work of the repo, they will then have a auction and sell the car.
There will be advertising fees, and more legal fees for the sale,

You can expect a repo car at a dealer auction to bring about 1/2 of its retial value, so they will get about 7,000 for the car.

So you take your 15,000 minus 7,000 plus most likely add back in about 2,000 or more in legal and repo and sale fees. You will still owe about 10,000 on this car that they will force you to make payments on.

So sell the car yourself, even if you have to borrow a little money if you can't sell it for its full value