Does IPhone X brought in Canada work in China with Chinese SIM card

SIM Locking is illegal in Canada, which answers the "SIM it was bought with" question. It will not SIM Lock to the first SIM Card that is inserted. That phone would have to be completely unlocked. You're correct on the warranty however.

My suggestion is to check the Tech Specs of the phone (Bands) and compare it to the provider on the iPhone X page. No one thinks of this.

I live in Toronto, Ontario. If he gets it from the Apple Store, it's completely unlocked. If he gets it from say Rogers, Telus and Bell (the three major providers), it is mandated by law, it come unlocked, if the OP purchases it outright. But to be on the safe side, get it from Apple.

as driver suggests, check the tech specs. especially keep an eye on LTE frequency bands.

don't believe the iPhone x is in a non sim locked version in which case it would only work with the sim it was bought with and as such only the canadian carrier and roaming in china with a canadian carrier would cost a lot of money

also the warranty is not international so if it had issues you would have to travel back to canada to have it fixed

also if china and canada use different LTE / 4g frequencies it would only do 3g in china.