Other users' activities can be viewed in the calendar depending on your role and position in the hierarchy of the organization.
Whether or not users can view other users' meetings depends on how the Reporting Hierarchy and Territory Hierarchy are configured in your organization. For example, the CEO can view the activities for all records, but a sales manager can only the meetings associated with their own subordinates' records and cannot see any information about their superiors' records.
Two sales managers can view each others' records if the Share Data with Peers check box is selected for their role.