> Curious though; can you ELI5 who actually pays the interest?
ELI5: Lots of people have business ideas that can make money, but often they need to have a small pile of money in order to make more money. Let's imagine you are such a person. I lend you money to start your business, then your business makes lots of money, and you pay me back. That's a good deal for you, but not for me. Your business might not work out, and you might lose all my money. I'll still lend you the money, but I want you to pay me back more money since I took this risk for you.
That's where the money paid in interest comes from.
Similarly, you might replace 'start a business' with 'buy a house' or 'buy a car'. You don't have enough money to do so right now, but you know that your life will be improved by having these things, so you borrow money to do them. The people lending you the money won't do it for free.
Less ELI5: Alternatively, the businesses holding this money for four weeks will estimate what the lowest amount they'll be carrying over the next 5 years will be, then just go and invest that amount in stocks, bonds, etc. Since their balance will never fall below that amount, the money is effectively an interest-free loan from Amazon that they use to make interest-earning investments with.