If you download a song you’re automatically a fan and it’s fan behavior and digimons have big fandoms. They operate differently than fandoms that buy physical albums. But they’re very much fandoms.
Causal ppl will not download a song in the era of streaming.
It's really hard to find information in English, but I found that Melon, Genie and Bugs have streaming + 30 downloads passes. But no artist releases 30 songs per month, so the remaining downloads are either bound to get lost or to go to someone else. I couldn't find if you can buy songs individually like on iTunes.
Edit: on Bugs you can buy a download-only pack.
Edit2: apparently on Bugs you can buy individual songs for 700 won.