Subtasks can be added for each tasks. It's just simple text with check/uncheck which can be done by tapping circle for each subtask. Subtasks status are shown on tasks list by numbers with format of (completed subtasks/total subtasks).
1. Subtasks on iOS version
2. Subtasks on macOS version (Integrated Panels)
3. Subtasks on macOS version in Subtasks Panel
On macOS, you can toggle to integrate panels into main window. On top menu bar, go into 'View > Integrate Panels' to toggle it. By checking it, Subtasks and Quick Actions will be shown at the bottom of right pane. Checking it off will let you open panels for Quick Actions (CMD-K) and Subtasks (CMD-/). Each panels can be moved anywhere and will stick to main window.
How it's stored
Subtasks are saved in notes field of Reminders data. It's shown on json form in Reminders app (Also 'Repeat after completion' and manual sort numbers are shown). You can delete these fields at once inside 'Settings > Advanced' but subtasks will not be shown if it's deleted.
Moving Tasks and Subtasks
You can drag and drop tasks and subtasks. Drag multiple tasks into a task and subtasks will be made into that task. You can also drag subtasks between tasks to make new tasks with subtasks' title. Moving between subtasks and tasks (Subtasks <-> Tasks) will always be copied.
Dragging subtasks into a task will move subtasks from one task to another. (Existing ones will be deleted)