I was digging to find it but gave up. if musical.ly is their main app then this is surely an overestimation of their valuation. I haven't heard a single good thing about musical.ly, but maybe that's just because it's not my crowd.
They run ads between the short videos, I would imagine that ad space is very valuable to some Chinese companies. I've never used Musical.ly however I imagine its similar just rebranded for an American market.