cdk bootstrap deploys a CloudFormation stack named CDKToolkit containing a staging S3 bucket, an ECR repository for image assets, and five IAM roles for deployment, file publishing, image publishing, context lookup, and CloudFormation execution.
The default qualifier is the literal string hnb659fds. It is a fixed constant chosen to look random, not a hash of your account, and it appears in every bootstrapped resource name and in the SSM parameter /cdk-bootstrap/hnb659fds/version, which holds the bootstrap version number the CLI reads before every deploy. Inspect it with aws ssm get-parameter --name /cdk-bootstrap/hnb659fds/version.
If you bootstrap with a custom qualifier, every stack must be synthesized with the same qualifier through DefaultStackSynthesizer or the deploy looks for role names that do not exist. Deleting the SSM parameter or the staging bucket without redeploying the stack leaves the environment half-bootstrapped, and deploys then fail with a lookup error rather than a message naming the real cause.