With the view settings in the top right corner of every project, you can decide how you would like to display Subtasks in your project.

Enabling the toggle "Subtask" lets you see the list of subtasks under each main task with an option to expand or collapse the list.

If you enable the toggle "Individual Subtask", all subtasks will be displayed as individual task cards. This option is great if you're working with a stage/phase-based process (like the classic "To Do", "Doing" and "Done" tasklists of the Kanban methodology) and want to move individual subtasks through the respective stages of your project.