They were on a mission related to sanctions in North Korea which is right next to china.

Well it's international airspace and nations can fly whatever they want inside international airspace regardless of the proximity to a nation.

Russians do it to Canada and the United States often but it doesn't make much fanfare because the parties involved handle it professionally

https://www.cbc.ca/news/canada/north/russian-bombers-buzz-canadian-airspace-1.5447833

https://www.stripes.com/branches/air_force/2022-10-18/russian-bombers-alaska-american-jets-7735230.html

why are Canadian military aircraft flying in international waters off the coast of China?

You mean, why are planes flying in international airspace?

Maybe grow up a bit, China. You’re behaving like a toddler.

Uh, they already do that. We generally don't buzz the planes and antagonize them

why are Canadian military aircraft flying in international waters off the coast of China

From the description of the video, should you care to read it:

Global News was on board the Canadian military aircraft reporting on the mission, which is part of Operation NEON, Canada’s contribution to helping enforce sanctions against North Korea, when the aggressive intercepts took place.

It said why in the video.

It’s part of our role in ensuring international sanctions are followed

It’s about shipping lanes to North Korea, nothing to do with China

How would we feel if they sent spy balloons directly over our countries?

Canada > China, they don't need permission to fly planes over the ocean