what creates SFTPCustomAuthLambdaFunction.zip? 03-sftp-endpoint stack fails

[onescore-agray]

I'm on step 3 in creating the 03-sftp-endpoint stack. The error is when creating the SFTPCustomAuthLambda resource.

The code for that lambda says it lives here:

      Code:
        S3Bucket:
          !Join [
            "-",
            [
            !FindInMap ["SourceCode", "General", "S3Bucket"],
            Ref: "AWS::Region",
            ],
          ]
        S3Key:
          !Join [
            "/lambda/",
            [
            !FindInMap ["SourceCode", "General", "KeyPrefix"],
            "SFTPCustomAuthLambdaFunction.zip",
            ],
          ]

But I can' find any instructions or shell scripts on what creates and uploads that zip file.

Any idea?