Sponsored promotional fanart is a thing, and the sponsor often can have the artists post the stuff at specific date with specific captions, which creates a feeling of a surge in popularity for bystanders. Once the tag/keyword becomes trendy, many artist wanna-bes would (have to) draw fanarts either out of affection/because they actually enjoy the game, or simply to ride on the popularity and attract followers for themselves - I'm not trying to badmouth this, and I feel for the rising artists, but this has been a pattern that I obvserved over and over again.

Personally I absolutely hate this game for both the gatcha aspect and the blatant plagiarism.

It was released as a highly-polished game for mobile, by a company that had experience in the space.

It didn't and currently doesn't do the external marketing tricks/growth hacks freemium games are notorious for.

Mobile, PC, and console all at once, as I recall. They really marketed the crap out of it and were on the necessary platforms to capitalize on it, too.

And with a game that had not only polished content, but plenty of it at launch.

I'm sure I'll get a lot of hate for bringing this up, but it's also worth noting that they definitely rode on the coattails of Zelda's success. So much of that game's identity is lifted wholesale from Breath of the Wild that it can almost be hard to tell the two apart in side-by-side screenshots. Again, none of this is particularly bad, but it definitely contributed to it's meteoric popularity.

Other than being cell-shaded, I've never understood the comparisons...

But I absolutely remember tons of people saying how alike they were, so I think you're correct about them riding its coat-tails.

https://en.wikipedia.org/wiki/MiHoYo ?

The plan of producing higher production quality games than most people expect on mobile, while also being cross-playable on PC due to Unity (nobody does that!) and cross-playable with friends on PS4 (pretty rare) worked for them.

It wasn't really out of nowhere. They had other successful games already and good fanbase, as also communicated the development of Genshin for some time already before release. But for Western PC-Gamers, they definitely came from a niche not many people followed at the time. They were more on Mobile and in Asia known.