Why do I see AIFF files although I used wav files when burning the CD?

A red-book Audio CD (CD-A) only contains an audio stream in 44,1kHz 16 bit format, which is separated into CD tracks by markers. It does not have a file system at all, so there are never any files on a CD-A, neither AIFF nor WAV files! It is just a special feature of macOS, that a CD-A is automatically mounted to a virtual file system, so that the tracks on the CD are shown as AIFF “files” even though these files do physically not exist on the CD. If you copy these virtual “files” to the hard disk of your Mac, the operating system will add the appropriate headers and create a new AIFF file with the audio content from the CD track.

On Windows, you will see .cda “files” instead which is again a mapping that is done by the operating system.