From this post by AWS [1]

AWS UI is built using react but they utilize Cloudscape Design System which is an open source design system for building intuitive, engaging, and inclusive user experiences at scale. It consists of an extensive set of guidelines to create web applications, along with the design resources and front-end components to streamline implementation.

This is a link to cloudscape website [2] and this is a link to its github repo [3]

Ref

[1] https://aws.amazon.com/about-aws/whats-new/2022/07/cloudscape-design-system-open-source-solution-building-intuitive-web-applications/

[2] https://cloudscape.design/

[3] https://github.com/cloudscape-design/components