There are 20 tracks on the left, each one seems to be compatible with 5 - 10 of the ones on the right. There would have to be 100-200 pre-made mashups covering all the combinations, so there must be some sort of systematic method for mashing up.

Oh; no there isn't [1]:

> Just cranked through em all for two weeks. Thought about automating it but it wouldn't sound as good

"When in doubt, use brute force", i guess. That said [2]:

> They're templatized, so phase 2 is to do it programmatically. Everything up right now was manual though :)

[1] https://www.reddit.com/r/Music/comments/5sgd4k/this_site_let...

[2] https://www.reddit.com/r/Music/comments/5sgd4k/this_site_let...