An EC2 instance combines an AMI, instance type, network placement, storage, and access settings. An AMI supplies the launch image; the instance type supplies a compute and memory shape. A key pair is one possible access mechanism. None of those choices replaces a security group or a suitable subnet.
Treat a launch as a set of requirements. Confirm the operating system and architecture match the application, then choose a type based on measured demand rather than its name. Some instance-type changes require the instance to be stopped, so plan downtime or replace capacity behind a load balancer. In this local account, the specified type is a training input. It doesn't represent a claim about current vendor pricing or availability.