In guesthouses you’ll definitely find and meet people who speak English and want to hang out. Bars and clubs too ofc. I heard pub crawls are a good way of meeting people too.

As for Koreans, from my experience and the experience of a friend, they usually keep to themselves. They might approach you in Hongdae or Itaewon and try to chat with you, but I’ve not heard from anyone that these situations ended up in keeping in touch with someone or even just meeting up for a coffee the next day. A friend of mine did get approached in Busan by a guy, who was also eating alone, so they chatted and ate together, added each other on IG, but that's about it.