MacBook buy in Canada , can be used in China ?

Yes.

Yes it will work fine but you will likely have to purchase a plug adopter or similar to mat the charger with the wall outlets in China.

The charger will wok with China voltage and frequencies so only a configuration adopter is needed.

The plug assembly on the charger and be removed and either power cable with proper plug used or y can get a ply the fits the charger and the wall outlet