I'm currently working on a project that integrate Zoho people calendar with google calendar and update all the leave events accordingly, I'm being able to do it using my personal account but my question is can I do it with a Google workspace super admin account? like, can I access all the primary calendars of all employees under our workspace organisation and update event using google calendar api by creating google calendar api credentials using the super admin account?
I haven't tested it using workspace account cause I'm not getting permission to test it using our company account. Even though I have created google calendar credentials using my official gmail account and I'm being able to fetch all my calendars including my primary one and update  events of my choice using gale calendar api.
Yes, you should be able to access your user's calendars via API and to do so you will need to:

Use a service account
Allow domain wide delegation to the service account (this way it
will have the permissions to impersonate your organization users)
Implement impersonation in your code

Basically the service account will impersonate the users that you specify and do the changes in their calendar on their behalf
Can I access all the users google calendar under my Google workspace organisation?