PCM & DSD are the two fundamentally different digital philosophies, and within both, you have different resolutions.
16 bit 44khz is CD quality
these are all increasing resolutions in the sampling.
DSD, double, and quad DSD are a similar representation of higher resolutions in the DSD path.
so that is the way that the digital music is created in its analog to digital conversion from the master tapes as it were. Now we have to save the file and choose a format of the file itself.
WAV - windows based, fully uncompressed file.
AIFF - mac based, fully uncompressed file.
FLAC - windows based, lossless compression
ALAC - mac based, lossless compression
MQA - newest kid on the block, promises lossless compression but delivers a smaller file size compared to FLAC and ALAC. After the file's oragami is unfolded, you can get a high res PCM file.
so, what should you buy? Depends on your gear. I have a dac that can handle 24/192 PCM. It cannot handle DSD. If I buy high res files for use today, I would buy 24/96 or higher. If I buy for the future, I can buy DSD and my software (Roon) can translate the DSD file to a PCM format that my DAC can handle.
As for MQA, I am streaming those from Tidal, and my Dac is unfolding them and they yield a high res file.
hope this helps!