cdflasdkesalkjfkdfkjsdajfd Why Terminal app needs to download 671MB from Internet?
It downloads a Debian Linux VM image.
cdflasdkesalkjfkdfkjsdajfd Why do not uses the system image to store this data?
Because the Linux environment is an optional feature, and not everyone wants to use it. It wouldn't make any sense to unnecessarily inflate the size of the OS image to include a feature only used by a subset of users. Also, the Linux image is hosted by Google, saving storage/bandwidth costs for the GrapheneOS project.