Some of us are systems/infrastructure engineers who have to build the intermediate layer. You can't just lay a dockerfile on top of a kernel and hope the system learns how to run it by osmosis.
Yes there are services like Fargate but they're not cost efficient for many cases.
The person was asking how they should develop their app to run on a particular host. If they need to run/deploy it, they can use the EC2 Instance Launch Wizard to set everything up in the console, log in and install Docker, use Docker.com to pull their container, and then run it.
Or, like you suggest, they could use an AWS service to manage their container, like App Runner, or Lightsail, or EKS, EKS Fargate, EKS Anywhere, ECS, ECS Fargate, ECS Anywhere, ROSA, Greengrass, App2Container, Elastic Beanstalk, or Lambda. There are plenty of guides on AWS's website on how to use them.
Cost is mostly irrelevant to the conversation, as you can run containers anywhere (other than, say, a CloudFlare worker); pay for any infrastructure you want and then run the container there.
Yes there are services like Fargate but they're not cost efficient for many cases.