1bit high speed modulation with shape dither to reject the quantization noise (rounding errors due to mathematical processing with only 1bit instead of 16) after the audio band. It was theoretically bringing 20bits equivalent of resolution.
With modern shape dither techniques, the CDA can be very close to the same but can’t reject noise beyond the audio band, so it puts it where our ears are much less sensitive (ie above 10kHz). Modern Masters are created with shape dither for CDA releases. This is the best practice (but of course depends on the studio).
The true advantage of SACD was the great care given to recording/mixing/mastering by the studios. At the very beginning some were putting a different master on the CD layer to that of the SACD layer. It was a bad practice, only to show off.
Today, all SACD I buy (yes they are still produced) have the same quality on the SACD and CD layer, and I can’t tell them appart (classical music).