I like touhou yes

It's a bullet-hell game that's been around forever and ever [started in 1996] made by one singular guy named ZUN. He's the one who does art, the game, and the music. Though there are other games, like spinoffs or fangames, that have other artists. The reason why you've seen it a lot is because touhou is pretty entwined into the internet IMO, a lot like how Homestuck is too. There's just a TON and I mean a TON of fan content created for Touhou, it's a pretty big fandom filled with a lot of people. Now getting into Touhou is a bit of a tricky thing, I'm not good at explaining it well, there is however a few different posts that can help. This tumblr post is good and talks about how you definitely should check out the Touhou Wiki, which is basically a must if you want an easy way to get into it. https://ciryes.tumblr.com/post/160198445545/so-you-want-to-get-into-touhou And here's the link to the Wiki https://en.touhouwiki.net/wiki/Touhou_Wiki

I find it best to just look at all the different girls in the cast and learn about her, then growing more into learning more about the plot of different games. At least that's how I did it. There's also a video [that's about fourty minutes] that goes into the Cosmology of Gensokyo [The world Touhou takes place in] https://youtu.be/4RtixP9TSKM

Reimu is basically the MC of the games, though there's other girls who you can play as later on. And Cirno is a very fun character [idiot] who is memed a lot. So it makes sense you would know those two. Touhou wiki in the fandom category says this about her. "Cirno is a quite popular character in the Touhou Project among fans. Due to becoming a meme, Cirno is one of the most popular characters of Touhou. Many fans portray her, jokingly or seriously, as a misunderstood child or even as an outright genius in disguise, or as a bumbling idiot who messes up what everyone else is doing. This is mainly due to fandom making references to her in non-related topics."

Flandre! She's also a very popular character, her theme is also well known, U.N. Owen was her? https://youtu.be/VIop055eJhU She's a little sister to the character Remilia, making them the Scarlet Sisters who live in Scarlet Devil Mansion. She's very cute, though strange, she doesn't go out of her mansion at all. People like to call her Flan because it's cute, and also a bit easier to pronounce.

Embodiment of Scarlet Devil, the game that has the Scarlet Devil Mansion take place in, is one of the most popular touhou games, so that makes sense!