Under Simple Permissions, visibility is set per record as you create or edit it.
Setting visibility
Accounts using Simple Permissions show a Visibility Permissions setting when creating or editing a record, which limits who can see it. The starting value comes from Default Item Visibility.
Tasks and events use a simpler, separate version of this setting: Public (visible to any user on the account) or Private (limited to the creator, the person it’s assigned to, and administrators). Administrators always see all records regardless of any of these settings, and none of them appear if the account uses Advanced Permissions instead. Users can set their own default for new emails, tasks, and events from their own User Settings; administrators set the account-wide default for other record types from System Settings.