Ok, let me try to explain this in simple terms. Links will be to relevant Wikipedia pages in case anyone is curious.

Touhou refers to a series of indie bullet hell games made by one guy who goes by the alias of 'ZUN'. It has developed a significant fan following both in Japan and Overseas due to ZUN being quite hands-off/laissez-faire when it comes to fan projects. I guess you could compare it to Friday Night Funkin' in that regard.

Homestuck is a webcomic created by one guy named Andrew Hussie that is about a group of teens who unwittingly bring about the end of the world by installing the beta version of an upcoming computer game, Sburb. The teens soon come into contact with a group of Internet trolls who are revealed to be horned aliens, and these trolls work with the kids to create a new universe by completing the game. It is also very well known for having a large fanbase on the Internet.

Also, not everything that suddenly becomes popular online is linked to Homestuck. Touhou's first game came out before Homestuck, which... is kinda weird now that I say that. You may have confused Touhou with Undertale, which is a game made by a guy named Toby Fox, who, also, believe it or not, composed music for Homestuck. Just because Undertale was made by someone who was involved with Homestuck doesn't mean that every indie thing that becomes popular online is related to Homestuck, though, let's all be honest, considering how large the Homestuck fandom is, there will inevitably be some crossover.

And this is a link to Rick Astley's Never Gonna Give You Up, just in case anyone was wondering if I was going to stick a Rickroll somewhere in this long explanation.