2 reasons for the meteoric rise:

Super duper aggressive marketing spend, akin Apple's iphone rollout.

A long going murmur about them using funny accounting to drive their valuation into stratosphere, and use it to raise money.

"Machine learning" and everything else all fluff.

I can pretty much imagine something akin to page rank for videos being used, with some perceptrons sprinkled on top.

That's the real reason. Bytedance spent millions, possibly hundreds of millions, to buy those users. You couldn't escape from those ads in 2019.