Amazon Prime is, and always has been, a subscription service costing £7.99 per month. Amazon does not mislead anybody about these facts.
If you buy from Amazon you will see a full range of products and the Amazon Prime ones are 99% next day delivery, with no additional coston the invoice. What you se is what you pay. These delivery and costs are built in and in part funded by the subscription. There is no limit to how many purchases you can make in any time period
Then of course there is another benefit available if you want to use it. That is of course the Amazon Prime television service which operates just like Netflix, and has hundreds of films, TV programmes both from the commercial movie/TV industries and Amazon Prime produced films and TV programmes.
Yes Amazon prime exists to make profit, in the same way as any other commercial business. I have an Amazon Prime account and frankly it is bloody good value.