"Browse" doesn't search anything - it just displays all of the tracks on your device in the order that you set as default, or chosen to sort them.
If you're looking to search, that will look at a variety of fields, including the song title, artist, album, and comment.
All of this relies on you having exported the content from rekordbox to your USB device. Any files not prepared in this manner won't be available through the browsing options, only the FOLDER method. As such, the file type doesn't matter - the database created on your export drive from rekordbox will be what the unit is pulling the data from.