Phases are a group of tasks and milestones and are not completed until the last take in the phase is completed. Milestone dates, etc. do not affect templates today, they are evergreen.

Milestones are groups of tasks. By default, a milestone's due date is its final task's due date. This can be overridden by a firm date if we and needed for project expectations/time to value.

A task by default is a single entity with a due date. A PM can add a start date to a task to add duration. The last task in a milestone typically sets when that milestone is due.