A Fly Volume just makes a chunk of persistent storage available to your app instance, it’s entirely up to you and your application to fill it with data. With the .toml you mentioned, assuming you’ve already created a volume nocodb_vol and launched an instance in the region with the created volume, there should be a mounted filesystem at the /data path in your application instance when it starts up. It will be empty at first, you can have sqlite create an empty file in this directory and then export/import any existing data into the running process.
If you really need to copy an existing database file (e.g., assisted.db in your example) over to your instance’s volume, some folks have had success with scp for copying database files over to a volume: Persistent Volumes/SCPing of Files