Where is the offline installation package downloaded?

Do you mean a standalone installer?

People usually use the CC Desktop app to install their apps – but if it's a standalone installer you're referring to (i.e., a direct download link), then that would go to the Downloads folder on your computer.

Just to elaborate on what  Test Screen Name  said:

https://helpx.adobe.com/creative-cloud/kb/internet-connection-creative-cloud-apps.html

Creative Cloud cannot be used offline.