As Aza mentioned, the objective of the Android Auto interface is to streamline functions to minimize the number of operations for motorists. There are other media players which do offer file-based navigation, but I think it's a poor substitute for a properly organized media library.
There are two separate solutions for your request: First, ensure all your media are well tagged so the media library can index your files. I have used a number of applications for organizing my media over the years; recently I have been using the open-source app Picard from MusicBrainz. Second, you can create a playlist in VLC from well-tagged files or (perhaps easier for untagged media) based on folders. Select your folder, navigate to the overflow menu "...", select "Add to playlist", and pick "Add this folder and subfolders". Name your playlist something unique for the voice assistant. Under Android Auto, navigate to Playlists and select the playlist you created; also try activating the voice assistant and say "Play <your unique name> playlist" to load the playlist. The Google Assistant may fail to find the playlist--it can be hit or miss--If so, check the search results on the playback screen and select the playlist by name.
Voice commands are documented here:
https://code.videolan.org/videolan/vlc- ... ote_330226