What Is Infrastructure as Code

Link to chapter - https://serverless-stack.com/chapters/what-is-infrastructure-as-code.html

The first line on this page says SST converts to CloudFormation.
The page " Set up the Serverless Framework" says: " While we use AWS CDK in SST."

So now I’m confused. Is SST using AWS CDK or CloudFormation?

SST uses CDK internally, which then gets converted to CloudFormation.