You cannot access other people's calendars via the Graph explorer, or delegate permissions in general. Only if the calendar is shared with you, or you have been granted Full access permissions on the mailbox, this might work.
If the idea is to be able to programmatically access calendar data on all your user, leverage the application permissions model. Read more here: https://video2.skills-academy.com/en-us/entra/identity-platform/permissions-consent-overview#types-of-permissions