Do Y Combinator startups hire outside US

I don't think there is a generic answer to this. It mostly depends on the startup size, their legal team and the team's. I have seen quite a few hire outside US, and then there obviously are startups outside the US that YC funds too.

Yes plenty of YC companies hire internationally. Usually we dont start doing it until we're a reasonable size and it may be from only certain regions or timezones.

My company hires internationally, plenty of others do too, and as someone else commented above, many YC companies are completely outside the US.