TLDR; It should be possible to get the files or folders from the Root/Home level in the HubSpot File Manager with the search API (already works for sub-levels) for example with the parameters parentId=null or path=“/”.
With the V3 Files search API you are able to get files or folders from a specific folder.
You are however not able (confirmed by HubSpot support) to get files or folders from the Home/Root folder of the File Manager. Meaning as of posting this idea, it is not possible to request the root level, and then continue down to sub-levels. Your only option is to start at a sub-level or to request all the files/folders in the entire HubSpot file manager.
This is needed for a lot of features that need to traverse the files/folder hierarchy. For example, a feature the mirrors the HubSpot web Browse Files feature.