CDA files is what you see when you stick a music CD into a PC CD player.
The actual "filing system" (if you can call it that) on the CD is nothing PC / Windoze compatible, and the music tracks are encoded with some weird encoding scheme. This was done many years ago for historical reasons, when CD players had no intelligence.
The CD writing software (which as mentioned can invariably accept MP3 as input format) takes care of setting out the whole image and writing it to the CD.