I have followed this tutorial and created a demo Spring Boot project using google cloud shell . I am very new to cloud and I am not understanding where are these files created under the project and how i can download the project
The App Engine Standard Environment in which you have deployed the application following the tutorial is a sandboxed environment and you cannot really access the instances and the filesystem where the application is deployed. Read this document to better understand how instances are managed.
On the other hand, App Engine Flexible Environment runs on top of Compute Engine instances and therefore you can access these instances via SSH, for example.
Take a look here and here. The two articles will get you started on the differences of App Engine Standard and Flexible environments.
I also recommend that you take a look at the tutorials to deploy Java applications on App Engine Standard and Flexible environments.