Group Sub-Task by Parent and then by Bucket Option

First, the plan needs to be a 'premium' plan (using Project Plan 3 or 5 license). Once you have that, you can click the three dots to the right of the task name, click on the 'Make subtask' option and it will nest that task under the one directly preceding it. If you want to reverse it, you just choose the 'Promote subtask' option. There are a bunch of other options available as well, to include cutting, copying, and pasting tasks, or inserting, deleting, or copying a link to a task, and adjusting task dependencies. Again, you'll need to have the Project Plan 3 or 5 license to access those features, and ONLY when using a premium template.