It definitely started in the US, but with tiktok culture it has spread! Ahmad is right that it is often used as a transitive verb (to slay something), like 'to slay a outfit/presentation/dance/etc. Now the object is usually removed and we hear 'slay' used on its own, and it usually refers to outfits/hair and looking very good. If there is a tiktok about 'Who slayed at the Met Gala' I would know immediately that the video would talk about whose outfits were the best/who looked the best. It is also understood that it is a risky or very eccentric outfit that other people might not look good in, but the person who 'slays' pulls it off and looks great!

This use of "to slay" is also popular in American drag queen culture. ("Girl, you slayed that lip sync!") You hear it a lot on RuPaul's Drag Race.

The other answer is correct. I just want to add that it is not very common slang among people over, say, 25 years old. It's more of a youth slang, at least in my experience. My experience is with US English. Once those young people grow older, it may become more normalized.

Yes, "slay" is a common slang word used primarily in American English but it can also be used in British English. It means to do something exceptionally well, to succeed or to impress someone. Here's an example sentence: "She absolutely slayed her performance on stage last night, the audience was blown away by her talent."