The Google Calendar client can work as a service without user interaction, so this is useful when you want to run a windows service, a daemon...
Google Cloud requires to create a Service Account (instead of OAuth2 credentials) to run this type of projects and the Google Calendar API requires the service account is using Domain-Wide Delegation to get the required credentials to access the calendars.
You can read more about how create Google Service Accounts.
Once the Google Cloud Account has configured with a service account and linked to a workspace email account using Domain-Wide delegation, you can configure the Google Calendar client to work with it, following the next steps:
After configuring the client, you can start to send requests to Google Calendar API without user interaction.