Hi, 

by default Odoo’s Project and Calendar apps are separate, so project tasks don’t automatically appear in personal calendars. You can link them by creating an automated action or a small customization that generates a calendar event (calendar.event) whenever a project task is created or updated ensuring assigned users see tasks in their own calendars. Alternatively, you can use the built-in Calendar View within the Project app to see tasks by deadline, though it’s not merged with personal events. For full synchronization, a lightweight custom module can link project.task and calendar.event models to automatically create, update, or delete events based on task start/end dates and assigned users. Since you’re not using the Planning module yet, this custom approach is the best way to get all project-related tasks visible in each user’s Odoo calendar.

Hope it helps