Google Cloud Service Accounts provide both identity and authorization to Google Cloud.
They are similar to user accounts. If you would like to do auditing or logging of actions with service accounts, you will want to use separate service accounts per user.
Service accounts are typically used for software applications to authorize their actions with the Google Cloud APIs. Service Accounts are using to issue OAuth 2.0 Access Tokens and optionally OIDC Identity Tokens. These tokens are what provides your application with authorization in Google Cloud.
My question is should every user get their own service account and
keys or should one service account be shared by all?
Yes, you should issue separate service account JSON key files to each developer. In the same way that you would not share usernames and passwords for computer systems, you would not share service accounts.
I have written a bunch of articles on Google Cloud Service Accounts that might help you understand how to configure and use them:
Google Cloud Service Accounts