whats is the differences in connecting to Compute instance (VM) through cloud-shell and ssh in gcp ?

Because they’re basically different in purpose and nature.

Cloud Shell is nothing more than just a Linux-based virtual machine, where you have popular tools installed to interact with GCP, and most importantly the tools in Google SDK including ‘gcloud’, ‘gsutil’ and ‘bq’.

‘gcloud‘ is the tool that helps you to to interact with GCP resources and services via API calls (instead of you calling those APIs manually). ‘gcloud‘ is not the tool for connecting to the operating system inside a VM.

‘ssh‘ is the tool that allows you to connect to an operating system inside the VM using SSH protocol, so that in the end you will get an interactive shell (Bash, Bourne, whatever…) to make things inside the OS.