So we have to use workflow to "move" the saved item into folder.
Lucky Workflow 2010 is still there, so we can use it to create list item in folder. However, we cannot create a list item from workflow 2010 instance, if the workflow is triggered by item creation.
The only choice is List Workflow 2013 + Site Workflow 2010.
When item is saved into list root folder by PowerApps form, it triggers the list workflow 2013 instance, which change the primary key field (to avoid the conflict with the forthcoming new item in the folder), then start the site workflow 2010 instance.
Pass ItemId to the site workflow parameter list.
In the site workflow, check to create folder if necessary, then create the item in the folder (specify folder name in the folder field), then delete the original item in the root folder.
The List Workflow 2013 instance will not be triggered if the item is created by workflow 2010 instance.
All these steps will be executed in less than 30 seconds during my test, so users need a little bit patience to see the result in the relevant folder.