Aside Apple (I guess?), where can someone *buy* music files in 2021?

A lot depends on your legal jurisdiction. 7digital is an option if you are in the US or the UK:

https://us.7digital.com/

https://uk.7digital.com/

Friendly reminder that CDs are music files, wrapped in a physical optical disc.

For downloading, take a look at this list from Wikipedia.

https://en.m.wikipedia.org/wiki/Comparison_of_digital_music_...

https://bleep.com/

https://www.hdtracks.com/

You can buy and download MP3s from Amazon:

https://www.amazon.com/gp/browse.html?node=163856011&ref_=na...

May not be available everywhere, depending on where you live

A few choices here via your part of the world.

https://www.pro-music.org/legal-music-services.php

Bandcamp!