Buildkite enables teams to scale CI pipelines using AWS-based Buildkite Agent clusters, allowing for horizontal scaling based on build demand, exemplified by Shopify's use of 50,000 daily build jobs. To simplify setup, Buildkite developed the Elastic CI Stack for AWS, a pre-built CI/CD stack that provides an autoscaling build cluster within a user's AWS VPC. This stack supports parallelizing large test suites, running Linux-based projects, and executing AWS operations tasks, while integrating tools like Docker and CloudWatch. Users can create multiple stack instances for different needs, with internal examples including a pipeline uploader, a builder with cached resources, and autoscaling runner instances. The stack is open-source, allowing users to build custom AMIs and run tools locally, and a step-by-step setup guide is available for getting started.