Only tasks from premium plans are stored in Dataverse. They are available via various APIs and applications to people with the right level of access and license. There are some limitations on how they can be updated, and there is a schedule API for that purpose. Applications such as Power BI can report on those tasks. Additional fields can be added to premium plans, or if you use a PowerApp you could extend the schema for additional fields. Plan 3 and 5 give access for customization - see the Project Service Description - https://learn.microsoft.com/en-us/office365/servicedescriptions/project-online-service-description/project-online-service-description